The problem here is that new features often involve some refactoring work. Then, if you didn't opt for feature X, the company isn't going to go out of their way to make and ship and maintain a fix for the old non-refactored version, so if you want fixes, there is no way to get them without feature X... Unless they are using feature flags of course, but then no fixes/updates are exactly "safe" for the combination of features agreed to.
Why can't they just maintain stable branches with bugfixes backported to them? You don't need feature flags for this, you can literally do this with just semver. Plenty of products support some number of previous minor versions but will backport bugfixes to them, and plenty of others have LTS releases that have a longer window of bugfixes still being made without new features.
If the claim is that there's something fundamental about car software that makes this less possible than literally every other type of software in existence, the burden is on the car companies to prove it. I strongly doubt this is the case, but if it is, I'd argue that the more prudent thing is to just _not_ keep adding features, because fixing bugs for the multi-ton behemoths hurtling alongside one another at high speeds sounds more important than literally anything that they could provide to the car that people have already decided is worthy of purchasing with the current set of features.
