The Mac is from a different time and has a very different user base than phones. By your argument the Play Store on Android makes it clear that the reason app developers put up with app stores is because of user patterns. On phones users are used to getting apps from the built-in store, so that's where they look for apps. Trying to sell an app via a website and APK download just doesn't work; users don't bite. There are alternative Android stores, most notably (outside of China) Amazon's and Samsungs. But both are wastelands, and devices with those stores typically also ship with the Play Store.

I don't think "app store competition" is going to be much of a factor for iOS post-DMA, but I do expect hobbyists and open source developers to congregate around AltStore or something and create a repository of free utility apps without having to pay Apple's developer fees.