Android has had side-loading for over a decade. Every major piece of software is still available in the Google play store (and many are also available in other alternative stores). It's not like there isn't already a real-world example of what will actually likely happen when side-loading becomes available in iOS.
Surely how a user defines 'major' depends on their unique perspective. I use a lot of scientific apps that are necessary for my work. As mercenary as scientific software houses can be, I expect they'll be gone to their own proprietary stores the instant side-loading is available and jack up the prices even more because the prices of the competition will no longer be visible. Time will tell.
Also, I take it the fee structure of Apple's store differs from others, so comparisons to the Android store might not be valid. Moreover, now that neither Apple nor Android can ban sideloading, there could be a proliferation of developer stores, with varying degrees of utility and consumer information. Imagine you are looking for apps that control left-handed wind-shifters. Now you just go to the app store. The format is standardised, including consumer feedback. With sideloading you might be forced to go to many store to try to compare apps. Under those circumstances an informed decision might not be so easy.
Not sure that regulating every aspect of how the app store works would be a "compromise" over simply allowing customers to obtain software from alternative sources. Side-loading only creates a market in which Apple has to offer superior services in order to compete. What you are proposing is far more heavy-handed.
I do not oppose regulation. However, i would like regulation to protect consumers, not EU companies that cannot compete without their government rigging the game.
All that would accomplish is making Apple's customers incredibly frustrated with Apple, and would give the EU ammunition to impose even heavier restrictions on Apple's business practices.
Admittedly it would, but it wouldn't be irritating as, say, popping up a message about unvetted software when side-loaded apps are used, slowing them down, pausing them at startup for a virus scan, or any number of ways Apple could sabotage side-loading.
Except that we have the Play Store as a concrete example of this not being the case.
Which means already consumers have a choice so this new law was not necessary, but in any case please see my comments above.
This still leaves users with access to only what Apple deems to be acceptable apps. It's not just about the fees Apple charges and the things they do or don't do to vet apps that they list on the App Store, it's about opening platform up to apps that Apple chooses not to allow for arbitrary reasons.
As I have suggested, it would have been possible to pass a law banning those arbitrary prohibitions in any app store and everybody lives. Preventing arbitrary bans on apps does not entail sideloading.