It's not about "never", as there is no point in keeping stable kernel API and ABI forever. It's about keeping them stable for a reasonable amount of time, in a way that administrators and 3rd party developers can actually understand and adapt to. Red Hat does this, btw.
Keeping API stable does _not_ mean one can't innovate - in particular, it doesn't prevent anyone from adding new interfaces, or new functionality. In the worst case, one needs to reimplement old API as a wrapper. It sometimes happens in userland, sometimes in horrible ways (errno symbol vs. threads, for example); but for kernel interfaces is seldom needed, and does not cause any "horrible problems", contrary to what "stable API nonsense" claims.
Regarding the UNIX heritage - yes, it's there, and nothing can be done about it, unfortunately. However, the whole "stable API nonsense" thing is about interfaces for kernel code, not for userland aplications.