Isn't this a case where the free market actually would account for that scenario? It's not like they have a car monopoly.
e.g. let's say my apple laptop breaks, even if I could fix it or get it fixed by a third party for cheap, I have to take it to the apple store to not risk losing my warranty completely.
The manufacturer's warranty can only be voided if the manufacturer can prove that the warranty claim arose directly due to the owner (or someone they paid). So if the owner changes their own oil and the rear bumper falls off? Warranty.