Even when I was selling through Kagi, I had a backup payment service running simultaneously (Plimus, then later FastSpring). I could switch between them in just a couple of minutes by changing some if statements in my PHP code. I even had hidden 'coupons' I could give to customers to redirect to another provider, if Kagi wasn't accepting their card for some weird reason.
Don't hard-link to a third party service. Abstract everything through your own site that you have 100% control over, so you can swap out the underlying services whenever you want.
I know you know the answer to this (i.e. move to another vendor, as you would for any vendor that goes away), so my apologies in advance for not understanding the point you're trying to lead me to.
I guess I don't understand the alternative. Go out of business?
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.
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.
Don't hard-link to a third party service. Abstract everything through your own site that you have 100% control over, so you can swap out the underlying services whenever you want.