For smaller devs 15% is a very good value considering marketing, card processing, etc. The scrape on subscription revenue has been pure greed on Apple's part and now here we are.
Apple could have headed this off by offering an App Store tier with a 10% subscription rate instead of acceding to the portrayal of the App Store as nothing more than a toll booth. Had the pre App Store status quo persisted, the mobile app world would be nowhere as developed as it is now.
That said, Apple provides a lot of value in the use of its APIs and I just can't get excited about sideloading. This will be a huge malware issue. It's strange that Apple can't seem to mention how much value devs get from access to APIs in their battle with Epic.
I think if you want to sell an app completely outside the App Store, you should write it from scratch and caveat emptor.