Well, Apple can still continue to offer it as a incentive to keep you offering your App through the AppStore.I'm an iOS dev too. Do you use Apple Maps? CloudKit? Do you submit more than 3 times a year? If so, $99/year for developer program is NOT enough to pay for these services.
If it was a choice, you'd likely need to pay additional to use Apple Maps, CloudKit, for each app submission, etc... because most of the top earners would switch to an alternate system. Apple would not be able to fund these resources if they lost a majority of the 30% cut. I as an indie developer do not accept this.
Right now I can release a 100% free app that uses Apple Maps, have 1 billion users use it, and not pay Apple a single cent other than the $99 a year. These types of apps wouldn't be possible if you were using Google Maps which the bill would cost six figures per month.
And those are just three examples. I've previously posted a long list of resources that developers get FOR FREE minus the $99/year.
Sideloading won't kill the AppleStore, and in that case other map providers will lower their prices or find a way to also offer it for "free or less", that's exactly what competition is.