With all the drama surrounding this, isn't the actual root of the problem that most US Apple users just can't be bothered to simply install another messaging app?
Pretty much. There are other messaging protocols that do basically everything iMessage does (and outside of the US, they're fairly ubiquitous). A better resolution to all of this would have been for everyone to just switch to Matrix/Signal -- heck even WhatsApp would be an improvement.
It seems to be a uniquely American problem that I can't get any of my contacts (on Android too, by the way, users there are just as apathetic) to switch to secure cross-platform messaging services. In a better world all this talk about RCS and SMS bridges wouldn't matter because we'd all have already abandoned SMS entirely.
For every 10 people I chat with, maybe 1 ends up falling back to SMS. Why do I want to tell the other 9 that we should all use Facebook? And how can I with a straight face suggest they are actually better than Apple?
Your argument works pretty well when the hate for Apple is greater than the hate for Facebook. In my experience it's mostly the other way around.