Not to defend a viewpoint I'm personally critical of, but this is a bit of a false equivalency that you're using that barely relates at all. The disdain for the movie is based on personal preference and expectations where as Sonic 4 not playing like the classics is an objective that those who obsessed over the most minute understanding of were able o see well in advance. Even when one can prove that the majority thinks something is ugly or likely you fail you have to consider that hat is a collection of subjective viewpoints that happen to align. The only fact that can be gleaned from that is that the majority, which can literally be defined as ≥50.0...1%, think it is ugly and that it will fail. It's not a fair comparison even if your intentions are in the right place.
So onto the problem in general
The problem is that you aren't taking into consideration how much affect that first opinion has on most people (my dad and I were just discussing first opinions the other day) and a number of the other factors that result in these outcomes. Most importantly however is that most forums like this one are made up of eth minority of people who will be consuming the product in any form. We are though particularly loud so we can seem like a lot more voices than we are. So these other movies succeeding for a number of entries is typically due to the silent majority continuing to partake to some extent since the declining numbers speak for themselves. For most studios whose primary purpose is to make money that is enough reason to let the creators go at it again regardless of critical or fan reception. That is one of the unique hurdles that the Sonic movie has encountered though, like New Coke their was enough public outcry and disdain that the prospect of making money looked threatened and the studio doubled back to avoid that and recover as much of the production costs as possible even if it has cost them more money in the process. Considering that let's break it down a little bit further
There are three things to consider here.
1) The marketing department is using millions of dollars to convince us that we should want to see it. They have failed at that in part to not putting out a great trailer which overrides the controversial design and because they had to promote a movie from a franchise whose main character in the 90s had a design rivaling Mickey Mouse in global recognition. That means you have a lot of people already having a pre conceived notion of what Sonic should look like that they've passed o to their children, nieces, and nephews. Thus you have a very large amount of people who are trained to think of Sonic in a particular way making the marketing team's job even harder since the trailer shows nothing outside of Sonic smashing an unrecognizable and unnamed even Dr Eggman/Robotnik that one could relate to the IP in any existing interpretation. It's a astronomically hard sell at this point.
2) You frequent mentioning of the test screening actually references back to saying see it to know for sure. Trust your own eyes in other words, and considering the one South American screening got me hopeful we would get something I found better than the motion poster and not less appealing to me, "seeing is believing" became a very good point of reference for me and a lot of others. it what I meant when I said that people have felt betrayed, and it definitely was not in a good way based on the response from Paramount to rebuild Sonic for the movie.
3)I wonder if you are familiar with the saying "speak with your wallet". If not it is a saying that generally means if you want the company trying to convince you to spend your money on their product that you don't like then don't spend any money on it. At this point most people contributing to this thread who don't like it have expressed a dislike of the live-action setting for Sonic, a human audience surrogate who will naturally take away from Sonic and Eggman to justify their presence, the lack of most game elements beyond use as Easter Eggs and a general avoidance of the source material, a generic plot of human with problems gets help from alien (in)directly while helping alien (part of why E.T is kind of a horrible thing to say to think of for this movie as again it makes Tom the main character and not Sonic), the movie design for Sonic, Eggman, and Eggman's robots, as well as the lack of that fantastical and surreal worlds presented in the games no less the cast of characters, and even Sonic's unusual never seen before lightning and EMP powers.
In short, regardless of how valid or not the complaints seem to you, to the person expressing them they are extremely important and enough reason to dislike the movie and express that in any form they can think of, up to and including not spending money on the movie.
So, while you're right that one can not, and should not pass final judgement on something they have yet to experience for themselves, they can say that it looks bad to them and that they don't want to give it the chance to prove them wrong. Now of course you also have to consider that sometimes when you see someone say "this is bad" that they simply aren't fully detailing their opinion as they are being economical with their time and are frequently action under the presumption that when they say they think something is bad and going to fail that everyone already understands all of eth additional information that goes into those words. And frankly they are not wrong to as most people talk to those they immediately know who typically do already presume all of that background information when the statement is said or something similar enough. When those presumptions don't exist typically that is when "why do you think that" should be asked. Unfortunately not everyone is willing to elaborate simply due to either an inability or refusal to believe that someone doesn't see something the same way they do. In those cases it is usually best to move on as there is no need to hurt yourself trying to engage with people like that.
My favorite example of a movie I found great but had horrible trailers was Get Smart starring Steve Carrell. Problem there though is that it mostly looked like they were being untruthful to the character of Maxwell Smart and not making him an elite agent like we was. In other words it was appearing unfaithful to the source material just like the Sonic movie. The big problem here though is that only Maxwell Smart seemed off (fortunately he wasn't, didn't even miss it by that much) where with the Sonic movie everything seems off or looks off. Bright fantastical worlds that can support darker narratives and themes. Nope, just the good ol' USA as Hollywood is wont to stick with. Oshima's iconic design that once rivaled Mickey Mouse in global recognition. Nope, instead a strangely human take on a cartoon hedgehog that doesn't even try beyond the most basic description of the character (I am kind of surprised that they didn't take inspiration from how Sonic's eyes were in the OVA and just give him a white raccoon mask to simulate his mono eye/eye mask). Eggman's colorful and inventive robots that highlight his playful and goofy sides. Nope, just generic Hollywood evil drones in black and red. And it goes on and on.
This of course leads us to your question of why not go in with neutral expectations and waiting to judge until after you've seen it with your own eyes. Well, if we look at the above paragraph there are very hefty amount of expectations that go with using Sonic and not meeting them is frequently met with disappointment (Paramount is alone here as SEGA isn't safe from that either). Now the problem that your idea of stay neutral is very hard to have as usually the only way someone can stay neutral is to be completely unfamiliar with something. If you don't know anything you don't know what to expect and can't have any expectations; only hopes of liking what you see. But you won't find that wit hSOnic here at least. This is a Sonic fansite so everyone here knows Sonic and has there own expectations for him so there is no way they can look at the movie and stay neutral. Even major supporters like @PeterPancake and @Myst are so overjoyed to see Sonic on the big screen that their expectations of the joy that should bring most Sonic fans ill prepared them for the reality that unfolded. They expected the love of Sonic that Sonic fans have would have made them love the movie but their expectations underestimated just how much people love Sonic. They love him to the point that they only want to see him done right and done justice, and unfortunately even if this movie were to put Shakespeare to shame it isn't doing Sonic right or doing justice to the brand. it is changing nigh everything and telling people that what they love can't succeed. Believe it or not but that is very easy to take as a personal insult because you're effectively being told what you like sucks and can't entertain anyone, yourself included. Arrogant hyperbole like that leaves people actually hoping for failure just to spite those who wronged them and prove that they had no understanding of why something was popular in the first place which has already been proven just by saying they have to change it or it doesn't work It's a bit of a messy situation where respect was lost the moment the creators didn't show any to what they were adapting and the fanbase of nearly thirty years. When you start by insulting people, no matter how unintentional, you lower people's expectations out of the gate making it even more difficult to have neutral expectations.
There is in the end a lot of negativity around this project because their is a lot of love from the fans and the creators of the movie, but the creators lacked respect and created distrust and lowered expectations. When those lowered expectations were exceeded in ways that were not well received it only made the original distrust stronger and the emotions built from feelings of betrayal that much fiercer. Unfortunately no amount of positivity on your end can overcome that instilled bitterness that you've been seeing here. just know though that it is so deeply ingrained because of how much everyone here loves Sonic, even if it is their view of Sonic that they potentially love instead as it is still Sonic.