AM travels a lot further than FM, all other things being equal, and the local frequency is signposted on road signs around the country.
The big three typical emergency broadcasts down under include wildfire (which have, and do kill motorists caught in them), flood, and cyclone.
The problem with 'narrowcast' mediums like mobile phones (inc. SMS), and landline phones, is that mass emergency alerts can overload those networks which can delay the message getting out, and it takes time to contact each endpoint in the alert area. The message they get is also likely to be short.
AM broadcast is immensely useful for non-overloadable continual updates on an emergency (or several emergencies at once). Though like narrowcast mediums, broadcast mediums also have disadvantages.
AM is used as part of a wider solution. Sometimes the only way to get an alert to people is via AM radio. It's much easier to mandate AM radio in vehicles, and never need it, than to allow manufacturers to drop it because 'reasons' and then discover we suddenly can't warn some members of the community because they relied on AM broadcasting (even if they didn't know it at the time).
It’s pretty easy to find a place where you won’t hear any FM stations reliably, but much harder to find a place where AM service is degraded to unusability.
It’s pretty normal for weather and traffic alerts to go out on AM radio in remote parts of North America.
I do agree that AM radio receivers are cheap to build and not a burden to require.
The actual burden of AM isn't the hardware it takes up, but rather the frequencies it hogs. Keeping AM indefinitely alive means we are reserving prime low frequency bandwidth for a dated tech for no real good reason.
Heck, I'd even agree that keeping the AM bandwidth dedicated to audio broadcasts is a desirable thing. However, I'd argue that it should be converted to a modern broadcasting format that can handle more than sermons and talk radio. Change it into a QAM digital broadcast with a fair bit of error correction and a modern codec and you suddenly can stuff a lot more stations into the same bandwidth, with a long range, and a crystal clear output.
AM radio travels farther for the same energy consumption, as a feature of the lower frequencies used. That means they can provide emergency information to a much larger area for a given power supply. AM radio also doesn't rely on huge towers since the waves can bounce off the ionosphere and travel as ground waves.
AM radio receivers are also very simple to make--check out the crystal radio!
Driving at night in the intercity mountain-west US, you can pick up radio stations quite far away. Not too long ago outside of Boise I was able to pick up some AM stations from Cleveland!
For starters, FM broadcasting is hardly more complex than AM broadcasting and any vehicle built in the last 50 years with an AM radio also has an FM radio.
I can't imagine a scenario where FM broadcasting is somehow unavailable, but AM is. But further, if AM is the last line of communication, we are in a VERY bad situation indeed, like apocalyptic. One in which I doubt any survivors are listening to the radio in their cars.
But even further, in an emergency, it's highly likely that car drivers aren't even listening to the radio. Like, unless it's something that everyone is made aware of with communications other than through AM radio. But if those lines are open, then why wouldn't communication be broadcast through those lines?
You may as well argue that we should make sure all homes have land lines or telegraph lines. After all, that's also a line of communication that could be used in an emergency.