Oh goody. I get to manage my own payment system. And have to worry about being tax compliant in every country and state. And deal with logins, and refunds and fraud and uptime and customer support.
Or just integrate other third party payment systems like Stripe, Square or PayPal. I'm pretty sure that they will almost immediately make their systems available on both Android and iOS when the bill passes.
Because a market is suddenly opened for business, alternative payment processors can now start offering systems that take care of this, and with different terms from Apple's. You wouldn't have to handle all of it yourself, you could simply sign on with one that provides you with service you like.
This is the free market competition that tech is supposedly in favor of.
The 15% pay cut is an industry convention, but is that actually justified by any financial reasoning? With actual competition in this space, business models can be discovered that could potentially lower costs. Not to mention, given the VC effect, there will likely be newcomers that try to undercut Apple/Google and each other with lower prices by burning investor capital, for a time.
People are already upset that their digital purchases are controlled by Apple. Wait until the startup that they bought stacks of software from goes bust and all of their app purchases are gone.
This current situation already exists with the App Store. There are apps that have been removed from the store that are simply gone forever, such as Flappy Bird, which was at least free. So how does this change anything? Just because Apple will outlive third party stores doesn't mean the situation you're describing doesn't already happen.