Better phones would be designed with upgradeable modems because the phone would be useful for longer than the modem is, but anything with WiFi can be connected to a hotspot.

I'm kind of surprised that isn't more popular in general, because the hotspot lets you connect all your devices with one plan for <$10/month if you don't use a lot of cellular data, e.g. because you mostly use it in your car and spend most of your time connected to some other WiFi.