There is a use-case for locked down hardware in an automobile: self-driving vehicles.
As much as you should have the rights to tinker on the things you own (and you should) things get a lot trickier when we start talking about the software or sensors that actually control your vehicle as it drives down the road.
It's going to be a complex issue with a LOT of debate, so I won't pretend like I can solve it in a single post. Suffice to say, lets not dismiss the entire concept of non-user-serviceable vehicles, in the long term. (though one thing I will say, that fact would have to be fully disclosed at time of purchase)
If liability is what you're attempting to justify with DMCA, let me tell you what will happen.
Can't maintain your own trees on your property because you might do it wrong and it could fall and kill a human. Gotta pay a licensed professional.
Can't clean your own kitchen because there's a chance you could do it wrong and kill a human via food poisoning. Gotta pay a licensed professional.
Can't install apps on your phone because you could do it wrong and be part of a botnet that attacks some corporation or country. Gotta pay a licensed professional.
Hope you see the common theme here. Don't think for a second it won't come to this if we open that greedy fucking door. Things won't get a lot "trickier". Those in power will lobby to mandate and control every fucking thing that they feel needs to have revenue driven into certain pockets. Not unlike what automakers are trying to do today.
As far as autonomous vehicles, don't worry. By the time we get to that point of mass autonomy, the entire concept of owning the car will be illegal. You will pay for every ride every time.