I'm not exactly sure the monoculture argument is valid for Windows vs. Linux though; Linux still dominated server usage while Windows was attacked more often, despite the fact that you could potentially reap a much greater reward infecting a server. Unix-based OSs were just inherently more secure for the most part.
While it's certainly true that Linux does much better on servers than laptop/desktops, I don't think it ever got to the >90% market share that Windows had on laptop/desktop (although admittedly that depends on whose estimates you look at). But servers certainly do differ in regards monoculture in important ways, relative to laptop/desktop.
It's also possibly significant that Linux on servers is split between Redhat and Ubuntu and others, who have some non-trivial differences in regards to security updates, etc.
None of which means that Unix-based OS's don't have inherent advantages in regards to security, just that not having a monoculture target on their forehead also helps.
