The sim card/internet plan supplied with the car isn't going to last forever (from memory, they buy a 5-year plan or whatever in line with the car's warranty period or similar. It might be possible to buy/subscribe to an extended plan, but I can't imagine why one would. I wouldn't expect any carmaker to seriously support the car past that anyway).

If you can share your phone's internet connection, then that works fine (and is probably ideal!), modulo my previous remarks.