Simple. As soon as a competing store is available that takes a 20% cut (or has less onerous rules about in-app purchases, etc.), developers will move to that store to make more money. Users will follow.
Amazon already tried this exact thing on Android, where the barriers are a lot lower. They've never had success with their app store outside of their first-party devices. Why? Because it's too dang hard, and I'm willing to pay an extra buck for Minecraft if I don't have to track it down in another app and worry about malware coming through the wall.
I'm all for a switch to install unsigned software, but only for development stuff. The App Store serves as a good barrier for 99% of users who just need to turn on their phone and use the darn app. The other 1% just need Apple to put a hidden switch to disable the protections ala csrutil in macOS and we'll be fine.
In our case, the demand being consumers, they are friction-averse and if you don't have a rock solid and unique value proposition, they tend to stay wherever they are.