A significant change that came with LTE is the use of OFDM as the base of the air interface, which means that cellular providers no longer need to allocate large segments of spectrum to one protocol or another. Towers that were upgraded from 4G to 5G didn't need to disable 4G or consistently split their spectrum between the two they way they did when upgrading from 2G to 3G or 3G to 4G, they can dynamically allocate subchannels in each mode based on the immediate needs of their users.

It seems unlikely that whatever technologies come in the future will regress in this way, so there shouldn't be a need for a hard cutoff at any point and I'd expect LTE devices to keep working for many generations to come.

I struggle to think of any system/scheme/protocol/spec/etc that promised this to have actually delivered. One can never predict the future.

Big enough that most carriers can't even launch 5G with its associated core network ("5G standalone"), and had to use the kludge of 5G + 4G core network ("5G non-standalone)

Right, so if I'm still rocking 4G only while 'everyone' else in the area is using 6G with some other stragglers on 5G, ...

However new technologies sometimes also bring new spectrum blocks like mmwave 5G.

You will keep receiving service until it becomes unprofitable to do so, same as always, but that time will be a lot further down the road than it used to be because there's basically no reason to care outside of congested cells where making the most of every bit of airtime matters.

If you're in a congested area, prepare for newer tech to be prioritized, often legitimately because the newer modes tend to be better at sharing the air and thus more users can be better served by prioritizing them on limited spectrum.

4G has become the new 2G when it comes to IoT etc.