Two way alarm systems with remote start have been a thing for pretty long and don't all require cellular connection. Some are just super long distance key fobs.

You don't really need connected cars for that. My car has no smart features but still has a remote start capability. It uses the car remote to trigger it instead of cellular connection.

I cannot remote start my car. If it's really hot or really cold, I go outside for a few seconds to start the car and then go back inside. It's really not that big a hardship.

Assuming you park next to your house a WiFi connection on the local network would be everything you need. Relatively cheap compared to the car would be a repeater to extend it for people like me who park 30-50m away I agree with you assumption that this is car manufacturers creating software based planned obsolescence. An open source framework would resolve this concern even over cell networks but defeats the entire point of also pushing power windows and seat heating as a service.