Performance regressions are different from ABI incompatibilities. If the kernel refused to do any work that slowed down any userspace program, the pace would go a lot slower.

Or be a lot uglier. See: Microsoft replacing its own API surfaces with binary-compatible representations to workaround companies like Adobe adding perf improvements like bypassing the kernel-provided kernel object constructors because it saved them a few cycles to just hard-code the objects they wanted and memcpy them into existence.

Microsoft's whole "Let's just ship all the dlls" attitude is a big part of the reason a windows install is like 300GB now.

Eventually you'd expect that something has to give.

Slow pace is appropriate for a mature kernel that the entire world relies on.

Not sure it is true anymore. I've encountered few userspace breaks in io_uring, at least.