This is a generalization. It still stands true that without critical reception that change might not otherwise occur. People will complain about anything just as people will also praise things that should not be praised. I like to comically think of it as White Knights vs Black Knights.
I don't argue there are a lot of complaints but that wasn't always the case. In fact, there was a time (mostly all of 2.0) that most negative threads got bashed to death by "white knights" (i use that as a general term, not an actual thing). The fact that more and more negative threads indicates a trend or possibly a pattern. Now what the trend is is definitely open to interpretation but there are definitely some concrete factors that contributed to the issues we see in the forums and in the game.
I definitely 100% agree with you that people that come on to literally say "xyz sucks cuz it does" are just adding more fodder. In a perfect world the threads would be a bit more consolidated instead of having many people making a thread on the same thing.
Hah that's easy. The Japanese forums will always have more weight cuz JP pride yo! But more seriously, really it has more to do with them keeping things to one thread. It also helps that their population don't have a massive disparity in comparison to to ours. Say the JP community was only 10-15% of the playerbase, maybe then NA would be listened to more. However, I will add that is just an opinion, I have no say on how well they listen to us in particular.