I think this misses the forest for the trees here. The platforms behavior here is a symptom and not the core problem. I think the following are pretty clearly correct:

1. It's your damn phone and you should be able to install whatever the hell you want on it

2. Having an approved channel for verified app loading is a valuable security tool and greatly reduces the number of malicious apps installed on users devices

Given that both of these things are obviously true, it seems like a pretty obvious solution is to just have a pop up that has a install at your own risk warning whenever you install something outside of the official app store. 99.9% of users would never see the warning either because almost all developers would register their apps through the official store.

But there is a reason why Apple/Google won't do that, and it's because they take a vig on all transactions done through those apps (a step so bold for an OS that even MSFT never even dared try in its worst Windows monopoly days). In a normal market there would be no incentive to side load because legitimate app owners would have no incentive not to have users load apps outside of the secure channel of the official app store, and users would have no incentive to go outside of it. But with the platforms taxing everything inside the app, now every developer has every incentive to say "sideload the unofficial version and get 10% off everything in the app". So the platforms have to make it nearly impossible to keep everything in their controlled channel. Solve the platform tax, solve the side loading issue.