There's no way Apple will capitulate on sideloading in the US. It's clear they are going to go every inch as far as the law allows to guarantee their 30%. They will strictly geofence sideloading to the EU, and they may even try to resist even more strongly by challenging DMA enforcement in court as long as they can before complying. And they'll probably pull more shenanigans like this to try to impose the 30% even on third party app stores.

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.

Why would they? Most iPhone users still won't install third-party stores, so, in practice, if you want to exist, you need to sell through the App Store.

Google allows third-party stores and side-loading, but they're still able to charge a 30% cut on the Play Store.

I personally make use of third-party stores and side-loading on Android, but I don't personally know anyone else who does.