So they broadcast on all AM channels, all FM channels, satellite & internet radio, and broadcast text messages.
A driver would need to have: AM reception, but no FM signal. No cell phone (or no cell phone signal) and be a habitual radio listener for this law to make a difference.
How many of those 100 drivers fit that bill?
But since this seems to be an FM radio addition, the law should've forced cars to have FM radio as well...
I do not know what large natural disasters are like in the US, but expect that would only become more frequent and severe, with a similar kind of failure rate across technologies like FM and mobile phone coverage.
As many countries would likely follow the pattern set by large car manufacturers affected by this bill, I am heartened by it and hope it passes.
All other stations (like most FM) have decoders that are tuned to and listen to other stations for the alert signal. They operate simply as relays in a tree. So no, AM is not going away, not now, not ever, lazy EV engineers be damned.