Obviously, physical infrastructure is a bit harder to switch, but I have three major wireline ISPs here, four major wireless ones, and I believe two satellite services are an option too. Meanwhile, most Google services have no meaningful competition that isn't incredible niche.
Letting users sideload an alternative store the Android way seems like a pretty reasonable solution to me. The experience is pretty much identical to installing arbitrary executables from the internet on PC (in that sometimes your system will pop up a security warning but you can continue despite it). The key difference being people are used to doing that on PCs and not on mobile devices.
But the only alternative seems to be mandating that app stores host competitors, which feels too specific to make for good legislation in my opinion.
But there's another problem which is the Google Play marketplace has a nation's worth of advertising spend to throw at getting eyeballs on it while something like F-Droid... doesn't advertise? And while there are good apps there, the level of polish is nothing like what you see at the top of the Google Play store. People are just so conditioned by shiny trillion-dollar tech that human-scale tech seems old/shady/etc and no amount of legislation is going to change that.
Okay, but tens of millions of other Americans have only one broadband choice (if they have one at all). There is clearly a monopoly issue there.