At this point, could some other open-source operating system topple Linux off its perch as the dominant open-source OS? There are others around, but they'd have to match Linux's stability, functionality and support levels before they'd be seen as a significant contender. That would mean attracting the same number of dedicated developers, and that's unlikely, in my opinion. All the developers who would be working on some other open-source operating system are already either working on Linux or on their own pet project. Those pet projects, lacking the developer effort and organisation of Linux, will move more slowly and will therefore feel like they're being left behind by all flavours of Linux, not to mention Windows and OS X as well.
Mokalus of Borg
PS - It takes a lot of effort to make a modern OS.
PPS - And in software, effort means time and people, which in turn usually means money.