I am willing to make a bet that even when side-loading becomes available, it does change much.

You can side-load on Android, and the Play Store's commissions are on par with Apple's.

The mistake here is believing that either Store is competing for developers. They are not. The stores compete for users, and they charge developers a fee for access to those users. Most users prefer the safety and convience of Play, or Steam, or Apple's iOS store.

So long as user's are primarily using Apple's store, either through defaults, habits or choice, Apple won't have to change much.