Producers should also be aware that a market will develop to circumvent these artificial limitations, and that will increase their own internal security costs, which seems like a battle not worth fighting for.
If you make a good, reliable car and focus on your customers' needs, you will thrive. These ideas of artificial segmentation seem profitable at first, but long-term, they turn customers off. I can't list the number of companies that I no longer do business with that I used to because they got a tad greedy or lost focus.
A car manufacturer that doesn't focus on these artificial software limitations will thrive (see Linux Kernel as example of growth in marketplace.)