I don't think it's effective way of making a change. Steam is thousand times larger than itch and even they had to comply. Expecting itch to suddenly have more power than such corporation as Valve is a bit wishful thinking, especially when itch.io hosted nsfw content for way longer than Steam ever did. So no, itch.io won't suddenly have more to say against the biggest companies in the world that control most of the transactions that ever happen. It's better to actually inform about the issue and force a legal procedure than bullying each site that has to cater to payment processors in expectation that suddenly those will start to care.
Viewing post in BOYCOTT ITCH
it was a joke. BUT if there was a private bank or credit union built with the right course to move not just this battle forward but any battle in the future forward that would fight to maintain some integrity. I'm sure bank runs would be the natural course. That's the reason bitcoin got so popular so fast. The idea of "decentralized currency" and even that might be a wrench in "collective shouts" agenda. the move to bitcoin processing.