I'd rather restate that problem: why do we consider monocultures bad, and based on that, why do we consider Linux a monoculture?
I've heard plenty of arguments against a Windows or OS X monoculture. Most of them amount to lack of alternatives when something doesn't work as desired, and lack of diversity for security purposes. The former simply doesn't apply to Linux: you have an almost excessive number of alternatives even for core OS components, and if you don't like them you can always make more (up to and including forking the kernel). For the latter, I'd personally rather focus on keeping one OS secure rather than two. Any other good reasons?
Let's not repurpose canned arguments summed up by words like "monoculture" without reevaluating those canned arguments against their new targets.