This depends very much on the position of that bit of userspace in the dependency chain, for me.
It largely depends on how serious the effects of a bad upgrade are and how hard it is to go back to the old component.
The kernel is unique there: there can be multiple kernel packages installed at once, and switching between them is as easy as selecting a different kernel on bootup.
With glibc (or with any other user-space library) there is no such multi-version capability: if the glibc upgrade went wrong and even /bin/ls is segfaulting then it's game over and you are on to a difficult and non-standard system recovery job.
So yes, i agree with the grandparent and i too see it in the real world that the kernel is one of the easiest components to upgrade and is one of the easiest components to downgrade. It's also very often dependency-less. (there's a small halo of user-space tools like mkinitrd but nothing that affects many apps)
Try to upgrade/downgrade Xorg or glibc from a rescue image. I've yet to see a distro that allows that in an easy way.
(The only inhibitor to kernel upgrades are environments where rebooting is not allowed: large, shared systems. Those are generally difficult and constrained environments and you cannot do many forms of bleeding-edge development of infrastructure packages in such environments.)
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