“So basically, ANY outside actor with only nominal expertise can break a component of those cars.”
The worst part is that the companies can force a ‘firmware update’ that sets off a code and forces you to go into a dealer. Luckily they only do that during slow times at their dealers and it is a way to smooth out the flow of customers, as they can do other repairs while they have the cars, rather than having to deal with the usual repair traffic where people bunch-up their repairs and overload the dealers, similar to the ‘lunch rush’ for anyone in the food business.
Because auto shops now demand $100+ 'diagnostic' fee for repairs. Of course you're going to combine several repairs per diagnostic fee.