Sounds like a fun experiment. I'd be interested in Apple allowing Patreon to test that theory by disabling purchases within the app. Is this really about access to a niche user group of users who would never sign up elsewhere?

Apple seems to be very invested in forcing Patreon to either not be in the app store at all (and given the limitations of web apps, this is a significant penalty), or offer exclusively Apple payments in the app. Then Apple goes a step further with rules blocking Patreon showing links to other purchase methods in the app. These are not the actions of a company that believes that its users can be trusted not to make a purchase elsewhere.

If these users would never give Patreon money by going to Patreon's website, Apple wouldn't be scared of a link to the website payment options inside of the app. But they are scared of that, because they know that many of their users would choose to pay less online if they were informed about the choice or if Patreon decided not to offer payment options in the app.

I think the fact that Apple is (according to Patreon) not offering a choice of whether or not to accept payments in the app pokes a lot of holes in the idea that iOS users would never use a website to subscribe.