I'm specifically talking about the following:
"Let's talk about X"
"It sucks. I hope they don't add it to the game."
"Then you are selfishly imposing your playstyle on others."
It's that third bit that I have a problem with. It's basically saying, "Don't participate in this discussion unless you like X."
The one part of "Don't participate in this discussion unless you like X." that I do understand is when someone makes a thread wanting to talk about a certain style of play and finding new and creative ways to make it work, and people show up to say "It already works fine this other way so don't even bother." In those threads, I certainly agree with the OP when they want to say in so many words "If you have nothing to add to the discussion, stop derailing the thread because it has nothing to do with you." Which is true... if someone asks for new ideas for X, then its polite if you're going to jump in to actually respond to the query being asked, and not just to say they're dumb for even asking it.
But it doesn't sound like that's what you're referring to either. It sounds like you're talking about someone who creates a thread where they say they hope for X to appear in the future, but then doesn't want anyone to respond to the thread disagreeing with assertion. In that regard, that person really needs to get over themselves.

This is a discussion about what
could appear in a future product, so giving the pros and cons of adding X to the game makes all the sense in the world. Now, we would HOPE that both sides would actually be able to discuss X by giving concrete
examples and reasons why their side is the more correct one, because that's how arguments are made and won. And I could therefore understand why one side might get annoyed at the other if the other side wasn't actually giving reasons to support their argument... they were just interrupting and derailing the thread by saying "Nuh uh! You're wrong!" over and over and that's it. At that point, I too would want to doofslap them off the back of the head for being an interrupting idiot who can't support their own argument. Because that kind of attitude just wastes time and energy, making it harder to follow the posts that actually have something to say.
Basically, it all boils down to that if you are going to participate in a thread, actually have something meaningful to say. Just don't be a cockblock because you don't like the idea and have nothing better to do with your time than stopping others from talking about it.