The old Yank Tanks are simple and easy to service with some basic mechanical skill and tools. Spare parts can even be fabricated if necessary. But ICE vehicles built in the last couple decades have become exponentially more complex. They're more reliable, but also more fragile and harder to repair. This is particularly a problem for proprietary electronic parts such as control modules. Once replacement parts are no longer available from the manufacturer it will become impractical to keep those ICE cars running.

They'll end of failing Smog checks and eventually be sold to Arizona and/or driven over into Mexico.

In Baja cars with California plates and long expired registration tags aren't too uncommon.

Hell, you'll even be allowed to go to the next state over and buy a gasoline car for the time being