Genuinely interested - what is using the FM frequency instead?

Well, nothing right now because it's still allocated for FM.

I believe he was saying what else could that portion of the spectrum be used for, if not for FM radio?

Correct. I don't see a huge amount of use for ~20MHz (probably less) of very long range spectrum. Potentially for smart meters and other IOT stuff, but current cellular networks deal with that pretty well (and there are other alternatives available). Plus I don't know much equipment that could use it at least out of the box so it is going to be fairly custom and very expensive.

It’s valuable precisely because it’s relatively low frequency, which means it works for things that need to punch through buildings.

That's a blessing and a curse though for interactive applications. One FM transmitter can easily cover millions of people - and 20MHz of spectrum isn't very exciting spread over that many people in my eyes.

So the way to use it more effectively is to build more transmitter stations at a lower power, but at that point you're competing with cellular networks which have access to probably 10x the spectrum and already have a load of infra built out to support that.

For example, if you used FM for LTE/NR it would be completely congested 100% with even just signalling overhead of the time apart from very rural areas (as suddenly everyone's phone that has no service because of being out of the reach of any mast suddenly gets it - perhaps because of thick walls etc).

Similar if it was used for WiFi - the transmit power would have to be so low to avoid interference it would have no benefit over current spectrum (2.4GHz doesn't work in most places for this reason).

And rural areas already have starlink - which has 2GHz of downlink frequency alone (plus they can add more somewhat easily, which is impossible for FM).

But on the other hand it's been used for audio for so long that the world is saturated with transmission equipment. I think I'll become a pirate radio band as soon as public stations stop transmitting. Of course you can police that but the 80s showed that it wasn't very effective. It's just too easy to build one with a few components.

Also there's often going to be a lot of interference from abroad, 100Mhz signals while VHF can carry far under the right conditions.

Any new systems will be digital, and I doubt they are that sensitive to highly localized interference. If the interference isn't highly localized the FCC will rapidly perform an operator attitude adjustment.

All the pirate stations back in the day were ultra high power AM stations operating out of Mexico.

There's still a ton of FM pirates today the FCC hasn't shut down: https://map.pirateradiomap.com/

One concept that is already partially deployed in the US is to broadcast stuff like digital traffic and weather data.