I think sysadmins and the server market would keep "Linux" as a whole from becoming a monoculture. But I think "Linux On The Desktop" would probably fare far better as a monoculture. That's my opinion, obviously. I'm generally an Operating system agnostic. I have OS X and Windows 7 dual booting on my MacBook. I'm writing this post from an old workstation running Ubuntu Jaunty (because I'm too lazy to upgrade manually and the in-place upgrade to Karmic has broken 2 of my virtual machines), I have an XP Pro workstation and OpenBSD server next to me, and a Sun Ultra 5 running Solaris on the top shelf. I play with this stuff ALL DAY LONG. Most of the other OSes I listed in this post are "a monoculture" and seem to be doing just fine.

Android is, in a way, a Linux distro that is growing strong now and is likely to grow even more now that Google is selling sexy branded phones running it. The same might be true in the future for Chrome OS (an Ubuntu derivative but a very different beast none-the-less). I don't think the evolution of Linux ends with Ubuntu.