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.