I'd imagine Google would plug any major holes in their soon to be closed garden, assuming that is their intention. So this and any other fix to the problem of 'install app through not-Google Play' that goes via technical means that Google can just cover up after a month or two doesn't actually move the needle any meaningful amount.
In the same vein, using adb isn't a real solution to that same problem for most people, since having to use adb is a massive jump in required effort that's going to leave all the normies behind, with only the super-dedicated willing to go through the hassle, and an equivalent amount of developer effort is going to be left behind as well, since their audience just got decimated, and they themselves might not even bother to develop something that even their dad or sister is going to bother/be able to install. Anything that's much more complicated than 'go to website, download thing, run thing, click your way through' doesn't solve for this.
The actual problem is to have Google not be knobheads about it, and the only way that's realistically going to happen is through the law, but that's not looking all that likely in my view.