You forgot to mention Google's upcoming developer verification push, which will prevent users installing apps - even from third-party sources - if the developers have not verified their identity with Google:
https://www.androidauthority.com/android-power-users-install...
Also, how sideloading works seemingly varies from vendor to vendor. On my Samsung devices I never had to fiddle with ADB or developer mode to install a APK from outside the Play store, I just download something, toggle crapware protection off, install, toggle crapware protection on and I'm done. Think it might has something to with all the customization work Samsung put into One UI (and just outright removing stuff from Google they didn't like).
But you are right, insofar that it likely is only a matter of time until Google's walled garden is as walled in as the one of Apple.