>It's that I, a user, cannot choose to sideload an app
That's only half the equation. I run android, but have no google account. I have always used a combination of aptoide, f-droid and manually downloaded apk's for my apps.
When all the covid apps were released and governments were encouraging people to download them, do you think they put the APK on a government website for people to download?
Nope, just links to the app store. Of course, most of those apps relied on google play services API's for a bunch of stuff, so it's not like they would have worked anyway.
It's not enough to merely allow sideloading. If the expectation isn't that mobile devices are a diverse ecosystem and that it's not good enough for developers to pick one store and bugger the rest, you've achieved very little. Most people could not be bothered going through what I do to keep my phone clean.
I've not yet found that to be the case, although I fear that one day it will.
I generally find that all you need to do is tell people their solution is not acceptable. All the banks in Iceland are trying really hard to convince people that phone based 2FA is the only option. However, I went to my bank and informed them that I don't take my phone with me when I travel overseas and I require another option. They tried a few other variations of "but you could use your phone this way", but once they realized I wasn't joking when I said "I often just don't have a phone with me, find me something else" it turns out that the physical 2FA tokens totally still work, they just don't like telling people.
Generally speaking, the people providing these things have a boss who expects them to make sure things run smoothly. They'll try to force you to use the app if that's the easiest path, but as soon as they realise that trying to force you to use the app will be much more painful for them than just giving you another option, they'll find another option.
It's that I, a user, cannot choose to sideload an app I want to use on my device or choose to use a different store.