Here's one competitor to Patreon. They were effectively shut down by VISA refusing to process payments for them. https://en.wikipedia.org/wiki/Hatreon

Here's another competitor to Patreon. They have been crippled by PayPal and Stripe refusing to process payments for them. They sound optimistic about finding replacement payment processors, and it's certainly their duty to their customers to try their best, but so far it hasn't happened. https://www.subscribestar.com/posts/5617

Elsewhere in this thread there are people pointing to indications that Patreon's banning choices were due to pressure from their own payment processors.

It's probably easy to create a Patreon competitor. If you want it to not get shut down, though, I imagine you'll have to implement ban policies at least as strict as Patreon itself. So maybe Patreon isn't a monopoly, but on the other hand, it doesn't make sense to say "It's fine if Patreon chooses whatever policies they like because, if the policies are bad, you can just start up a competitor with better policies".

The relevant monopolies/oligopolies seem to be the payment processors.