Most customers are probably going to assume your a con artist if you don't use the official IAP.

When I use mobile apps I like being able to do all my spending in one place. I want to be able to go to subscriptions and cancel everything I don't need at once.

Just yesterday I had to manually stop a PayPal payment renewal since the merchants cancellation process doesn't work ( 2 emails to customer service and I get the vibe this is intentional).

That's not something I want to have to keep doing.

I can imagine Epic being able to convince people to use a 3rd party payment provider, but that won't happen for smaller studios.