On grid cars don't tend to stay that way. My 2013 Ford was built with a 2g modem, a recall replaced that with a 3g modem, and now the 3g modem has no one to talk to. My 2017 Chrysler also has a 3g modem with no one to talk to.
A malicious person could standup a fake 3g network, I guess. But LTE has strong mutual auth, so cars with 4g modems will be very hard to attack once 4g is dead. OTOH, 4g and 5g can more easily coexist: as I understand it, 5g can run with 4g compatible control protocol, with some slots 4g and some 5g depending on the needs of the mobile stations nearby, 2g and 3g needed a block allocated, so once the minimum size block was no longer well utilized, it's a waste of spectrum. This may mean 4g is kept alive a lot longer than 2g/3g.
A malicious person could standup a fake 3g network, I guess. But LTE has strong mutual auth, so cars with 4g modems will be very hard to attack once 4g is dead. OTOH, 4g and 5g can more easily coexist: as I understand it, 5g can run with 4g compatible control protocol, with some slots 4g and some 5g depending on the needs of the mobile stations nearby, 2g and 3g needed a block allocated, so once the minimum size block was no longer well utilized, it's a waste of spectrum. This may mean 4g is kept alive a lot longer than 2g/3g.