Something like that is unlikely to happen because the US is not — for now — an authoritarian state.
Replace “FBI” and “Biden” with their equivalents in various other countries and yes, it’s absolutely plausible.
There is also international perspective: once this mechanism is in place, forced by US, it will become normalized in several years. Then China will have much better leverage to implement their own scans. Once China does it, every powerful state does it. And after that every third world hole will scan for everything.
what's stopping them from doing that now?
Because Apple does not currently have code for scanning local devices for illegal images, and would have to implement it, which is a much higher bar than having already implemented it and just needing to add a few hashes to a database.
but they do have it. pretty much all image hosts are already doing it as well