I think the choice is between using a solid platform like the App Store or something like it, or building your own using tools that won't go away either. Stripe, PayPal, things like that.

It takes a lot of effort to build out your own store and get it right, this is no small task, which is why the 30% fee charged by these stores is not necessarily the biggest obstacle.

These intermediaries add considerable risk.

I'm not even sure we can assume Stripe won't go away, and Paypal has shown itself to be anything but dependable (sure it'll accept a payment but will you definitely get the money?)

Edit: Sorry I know those were just two examples - I'm not trying to refute your point

They're bordering on "too big to fail" at this point, but if they did go, they are just a payment provider. Switching from Stripe to PayPal to Square or whatever for credit card processing is a heck of a lot easier than switching from the Apple App Store to your own hand-crafted store.

For example, you can bang together your own store with Magento, Spree, or any number of other ecommerce store kits and pick one or more back-ends without a whole lot of sweat. This presumes you know a thing or two about web development and security, which is not necesarily a given for any group of developers. If you do know that, the difference between Stripe or PayPal is a few lines of configuration and API keys, if that.