Google is facing antitrust scrutiny for the amount of friction they have for their sideload process. I don't think that will survive this law in its current form either.Personally I think the danger of individual apps trying to go it alone pulling their user bases to a sideloaded solution seems unlikely. We’ve seen how little of that actually happens on Android.
The way sideloading currently works on Android is that it's not allowed by default and even when allowed, does not permit the installed app to itself install anything else. That makes self-updating or alternate app stores impractical. That friction is intentional and like Apple I think it's there for a combination of good and bad reasons, and like Apple, Google has allowed rent seeking to become entangled in the user's interests. Make it easier and I think it's going to become far more common.
There's drawbacks to that and will very likely result in BonziBuddy-type malware that installs unwanted components without user consent. This is one of the reasons I suggested beefing up sandboxing with virtualization technology. But like Apple, I think Google is going to be very late implementing actual considered engineering responses to this, and may indeed want the legislation to backfire and so will potentially simply allow the problem to fester.
But can't you install the Amazon App store on your android phone?