Nobody expects the spanish plonk squad!

You still don't get it. Incompatible changes in udev did happen. _Exactly_ because somebody thought that there is such a thing as application-private kernel API. And it was a fucking disaster.

These days any kernel developer who would even try to pull off something like that would get painfully educated on the reasons why You Don't Fucking Do That(tm).

It doesn't matter how many programs use it. One boot-critical is more than enough. Quite a few of us avoided udev for years after that stunt, exactly because it made our lives very, very unpleasant *and* udev developers tried to pull off that kind of indignant "but it's just a private API, how can you say that we don't get to change it at whim?" shit, making everyone suspect that they fundamentally didn't get it.

We can keep several kernel images and boot one of them. Easily. We can not do the same with the userland programs. And you don't get to assume that your Great! Shiny! Change! renders all previous history instantly obsolete and uninteresting.