glibc, which has had ABI compatibility for decades?
There have been plenty of build breaking changes over the past couple decades, generally they happen for very good reasons and only affect niche usecases.
So existing binaries will keep working?
Not necessarily if you go back far enough to have nss problems :)
That's an incredibly ignorant claim. Just run "git log" in glibc, it won't take you very long to prove yourself wrong.