On the one hand, the number of people using non-Linux POSIX compliant OS's have dropped significantly in the last decade. As a percentage of the installed base, either by machine or by user, what's left? The *BSD's, where I'm not sure there is hard evidence that they are growing even by absolute numbers, never mind as a percentage of the total market. Solaris, which Oracle is busily killing by increasing support costs to the point where people are quickly bailing to other alternatives --- in the end I won't be surprised if aside from the one or two big iron databases servers, there are very few other Solaris installations left. AIX? HPUX? Don't make me laugh.
The only POSIX compliant OS that is growing is MAC OS X --- and Apple isn't afraid to add new incompatible features to their kernel, and then release software which only runs on MacOS --- and usually people don't wag their fingers at Apple complaining that they are releasing non-portable programs. Instead, Apple is usually lauded for keeping a solid eye on usability and writing programs that have a deep sympathy for the end user.
So why is it that a lot of people are kvetching at GNOME for wanting to use kernel-specific features in the name of improving the user experience, when the same complaints aren't leveled at Apple? I think there's a bit of a double standard being applied here....