Does it really matter if the vehicle is not connected to the Internet in any way?

I mean, if an attacker has physical access to the insides of your car, all bets are off and is now his vehicle.

They might not have the internet (although some do via various methods) but most will have Bluetooth which could kill the console in the best case. There has been cases of people getting access to things like the door locks and the brake system.

Remember it's not just the internet that makes you susceptible, it's any network.

The issue is that an attacker could gain access to the inside of your car only briefly, but leave a device that continues carrying out the exploit

And do what exactly? make the car crash? If you want to do that just cut a break fluid line, don't even need to open the car for that.

I mean, I take your general point, but I'm almost certain that 99.9% of people breaking into cars do it to steal them, not to install a virus that will track your location or make your car crash. Both of those can be achieved trivially without even opening the car.