Most telco execs would sell their own mothers before offering reasonable data plans - that your car comes with one for free should be very telling
I used a bypass harness to avoid losing a speaker and the car hasn't developed any fault. I wouldn't buy a car without knowing in advance that this mod is possible. There are plenty of tutorials on how to do this for popular makes/models. It may be as simple as removing a fuse, or you might need to disassemble part of the dash to physically unplug the modem.
For example if you look at where Tesla and co. are going, they are looking to reduce the amount and tickness of wiring harnesses - and they did so by creating a proprietary fieldbus that communicates between proprietary controllers that handle multiple functions, so they can multiplex everything under the sun into a single bus.