They'd have to require all current subscriptions be cancelled and the re-upped with the new payment processor, no? That's gunna be really costly

But then again to avoid a 30% fee.. probably worth it

Because Patreon doesn't want to do that. They could.

_in the US_

They don't have to "cite the Epic case", it's just functionality available to everyone now. Your app is no longer blocked from approval for including an external payment provider.

They'd actually have to do it though and that could lead to a large loss of revenue for themselves and their subscribers.