If the payment processors really cared about children, they would be banning companies that sell guns, not video games with boobs in them.
Maybe they will next, maybe not. What does that have to do with them taking this first step? We have to evaluate the merit of this action, on its own.
Personally, I think that opening Steam's "New and Upcoming" and seeing nothing but low-quality porn games is bad. Bad for steam, bad for gamers, and bad for children.
Is this something that happens when you have a registered account? I don't have a Steam account. I opened https://store.steampowered.com/explore/upcoming/ in a private browser window and I see only one one game out of 20 tagged with "Nudity" / "Sexual content".
I don't think it's "whataboutism" to ask why they don't do something that helps society with that power instead of using it to censor art they find personally distasteful.
I don't see any destroyed industries.
Payment can be processed many ways, games can be bought from multiple store fronts, and there are still pornographic games on steam... so...