The fact that a certain, common kind of person in fandom thinks there's any valid argument for unsolicited (and unwanted) criticism on fan content is, in itself, a tell. In general, unsolicited criticism is bad—or, at the very least, misguided—because the agenda of the "advice" giver in the first place is to try to control or change the other person, rather than to try to understand them. That doesn't really change just because we happen to be talking about writing, or art, or fandom, or any combination of those things. Criticism is a part of art, but artists only need so much of it to grow, and it's reasonable to assume that they might only want it from people whose opinions/perspectives align with theirs in certain ways rather than from strangers online.
For the people struggling to grasp this truth about unsolicited criticism in general, there's not much to be done. They have to realize it for themselves when someone finally explains it to them in a way that makes sense to them, or enough people walk away from them/start leaving them out of the loop.
For the people who do grasp that in general, but suddenly turn into the insufferable "I'm just honest!" guy when it comes to a specific space/hobby, the ass is out, I fear. Both cheeks. To this person, people in fandom are the equivalent of kids who Naruto run in the hallway and should expect to have their head dunked in a toilet. Any suggestion this might be bad is met with Entitlement Tantrums because they're bullies, and beyond any internalized mistreatment, it isn't any deeper or more enlightened or more interesting than that.
Hopefully someone else can find freedom in this realization like I did.

















