They don't though.
This has been shown time and time again.
Those fans have a good idea of what they, personally and specifically, like and dislike, but not the larger group of people who will go and see a movie or play a game or the like. "Serious fans" have very often predicted something would a hit or a flop based on their personal tastes, and been completely wrong.
Your specific examples don't support your position. Let's look at them.
Sonic - This was immediately widely mocked. It wasn't "Sonic fans" who said it was wrong, it was literally everyone who had ever seen Sonic before, hell, even some people who hadn't but were repulsed by this creepy thing. Nobody liked the design to the point that there's been some (not entirely baseless) speculation the whole thing was a publicity stunt - because it was so far out from release it was very easy for them to "fix" it, and doesn't seem to have cost the movie a lot to do so.
So you didn't need "serious fans" to tell you this. The "serious fans" just agreed with everyone else.
4E's marketing - Again, you wouldn't need "serious fans" to tell you "insulting all the previous editions is a bad idea". You just need to not be in an echo-chamber, which apparently WotC's marketing department must have been at the time.
"Serious fans" can also get things very wrong by contributing to echo-chambers too. For example, when Blizzard wanted to introduce Real ID which would show your actual, RL email address to basically random people in WoW (potentially including your RL name - most people's did, and at the time it was extremely painful to try and change the email associated with your WoW account, and it kind of still is), they sold this as "Well everyone is called stuff like John Smith, that's not going to be a problem!" because to them, "everyone" was the extremely white, extremely male echo-chamber at Blizzard. And a lot of "hardcore fans" defended this, because they too were mostly extremely white and extremely male
A more complex example might be the Classic servers for WoW. Originally WoW's director said, famously "You think you want it but you don't!" re: Classic servers. Blizzard later changed their mind and created those servers, and initially they were a big hit (like 4E, actually), seemingly proving the previous director wrong. But the numbers have declined extremely hard, and Classic servers are now mostly just a weird place full of horrible people and the worst behaviour you'll find in WoW. The earlier director was absolutely right in that they aren't generally popular with WoW's audience as a whole, and the only people who seem to like them are a peculiar species of "hardcore fans" or "serious fans" who are quite unpleasant to be around, quite anti-social (ironic, given Classic requires more social behaviour). But he was wrong in that that specific kind of fan, and basically nobody else, does like them. In a weird way they serve as a kind of useful "self-exclusion zone" by attracting some of the worst of the worst to them, but they also unfortunately drive away returning players who think they'd like to play early WoW again, only to find it's full of incredibly obnoxious people who want to tell you how bad and dumb you are (which, like, I played early WoW, it wasn't even 10% as bad as this).