Furthermore, if Patreon goes then a lot of indie people are royally screwed. So many demonetized Youtubers or people at risk of being booted depend on Patreon.
Having members on YouTube doesn't impact that at all. The members will pay and discover, same as if they were on Patreon. Non-members might have a harder time discovering, but it's not like it's easy to discover content through Patreon either.
Only if the Visa/Mastercard duopoly approves. It's not a real market.
Cardless Subscriptions are also here (disclaimer, that's my role at a large open banking company) although some more legislation needs to be done before they can be used for ecommerce
For the Creator, even if you exclude the things that interact with Patreon to do the benefit-management bits, you have to also do any KYC/Identity verification bits, re-setup account transfers.
Then you need to reach out to all your supporters and convince them to move over. They're going to lose some percentage of those people -- how much is going to depend on the creator and the fanbase. Some people are just going to ignore it, every single one of them though are going to re-evaluate that math in their head as to whether they really want to continue supporting that person.
Patreon also has major brand-awareness. Someone telling you to move over to SubscribeStar, Liberapay, Buy Me a Coffee or whatever might get a "This seems like a scam" type reaction.