And yet, we're on a comment forum full of people who would be ecstatic if Linux had 95% market share, those pesky BSDs remained (or became, in OS X's case) a distinct minority, and the not-really-POSIX platforms like Windows just disappeared. (Myself included, in fairness.)
A lot of the usual reasons to worry about monocultures go away, I think, when you have a large public development community and when there are independently-driven forks in use with somewhat different code, especially for newer code (the distro kernels, in Linux's case). So I'm not too worried.
What worries me more is, for instance, Apple's requirement that everyone writing a browser on iOS must be using the system WebKit (the so-called Chrome for iOS included), since that keeps all power to patch that web engine for an entire platform in one entity's hands. Opera for Android and Chrome for Android both using WebKit, using different forks of WebKit, not so much.