I understand your predisposition against DLC/Microtransaction. They can be predatory and limit games, however there are reasonable business models based around entirely optional content that people actually want to purchase.
In an age where people won't play a game because it costs 1 or 2 dollars, or it isn't on 75% sale, there are certain small games that are wholly unmonetizable without optional paid content **after** your player enjoys your free game. Are you sure those games shouldn't be made? Should they only be made by hobbists without any hope for compensation?
I haven't played AC:O, but I expect that people are mostly mad that a game that used to be premium, buy-once is now both buy once and microtransactions. Undoubtedly those transactions also affect the gameplay itself, so it's like you're continuously paying to play. I agree that games shouldn't be monetized so aggressively, but don't you think it's a bit unfair to compare AAA games to indie games? And moreover to remove an entire source of income for everyone based on "the big boys are being bad"? Why not just lobby for people to not play AC:O? If you think the series will collapse, then maybe you don't need to lobby for anything and people are smart enough to not play with that kind of monetization anyway.
Finally, consider this: A microtransaction is mostly data already part of the game. My game already contains all of the hat sprites I want to sell. I simply can't sell them well on itch. Itch hosts thousands of games that may or may not be like this. Additionally, itch maintains the receipts for every game purchased on itch, which amount to perhaps a few hundred bytes per purchase. If itch can't host the additional bytes for a game's microtransaction receipts (which itch deservedly will take a cut of) then what makes you think they will be able to continue hosting my (and thousands of others') free game (tens of megabytes each) which they get no money from?