Not to say I would be a great fan of Linux, but I think at this point it's quite clear it's the huge winner of OSes. Nearly all servers run on it nowadays, I think all relevant smartphones (not sure though, correct me if that's wrong), many desktop/laptop computers and even Windows is integrating more and more with Linux.
With such a dominance there is not much point in syncing changes of the standard with other OSes. The moment it works different in Linux that is the new majority standard.
And while a lot more stuff is changed all the time than would be necessary one also can't expect things to stay the same for another 100 years just because they have been like that for a long time.
Mostly thanks Torvalds accepting just about anything into the kernel, not wanting to play politics if someone rolls a kernel with some drivers that are not GPLed, and keeps a strict API/ABI stability.
Above the kernel it is a wild west, with each "platform" implementing their own stack for anything beyond the core-utils (and those are more likely to be busybox based than GNU based for political reasons).
It's not the first time I hear it, but what is it supposed to mean? Is there a single thing that can be called "linux userland"? Also, I doubt that someone combines Linux kernel with Windows XP userland. So in some regard all userland tooling running on the linux kernel might be considered "linux userland".
If I understand the term userland correctly it is a term from kernel developers to say "this is code that doesn't run in the kernel", so it might not be helpful to any popularity discussion at all.
With such a dominance there is not much point in syncing changes of the standard with other OSes. The moment it works different in Linux that is the new majority standard.
And while a lot more stuff is changed all the time than would be necessary one also can't expect things to stay the same for another 100 years just because they have been like that for a long time.