I think the issue with shippers from a creator perspective is less them discussing with other fans how they'd like a story to go or imagining alternative ways it could have gone through fanfic or fan art, and more that some of them seem to want to tell the creators directly how to make their story.
Like imagine you're making a cake, a delicious carrot cake and somebody you've never met comes up and says, "hey, you should make this a chocolate cake", and you say, "sorry, I actually really want to make a carrot cake", but they keep insisting, "NO. You're doing it wrong, nobody wants this carrot cake, a chocolate cake would be better." and you're like "But this is MY cake! I'm making the cake how I want to make it, I didn't even get out the ingredients for a chocolate cake! I don't know how to make a good chocolate cake!" and then it escalates to, "IF YOU DON'T MAKE IT A CHOCOLATE CAKE, YOU'RE A BAD BAKER! AND YOU'RE A BAD PERSON!" and you wonder how it got like this from your original innocent notion of "ooh, I think I'll make a tasty carrot cake!"
In most cases, it doesn't get that far, but it's just generally frustrating when you're making your own creative work, and usually as a solo creator this will be a labour of love made because you wanted to make it, not out of some corporate mandate, just purely "I think a comic/story like this would be cool and I want to make it" and suddenly somebody is telling you how to make it. Especially when on platforms like Tapas or Wattpad, there's really not much preventing literally anyone from making their own work that does have the kind of ship they're interested in instead of trying to backseat drive somebody else just because they already put in the work to build an audience. If it's a big corporate production, I kind of understand more, since marginalised people are often locked out of making their own works in print or for TV etc. So lack of representation, hetero-normativity and queerbaiting is a legitimate and frustrating issue there and it may feel like the only way to have a voice on the matter. Still, some fans have definitely taken this too far.
tl;dr, most small creators are just doing it out of love and you can always make your own thing, so if you're not giving them objective critique they actually asked for, let them enjoy making what they want to make, even if you'd prefer if the pairings were different.
