The Linux kernel isn't a monoculture yet. For example there is no indication Apple or the BSDs are going to stop developing their kernels nor is there indication Windows is dropping NT and most anything can/is easily moved between them (particularly the *NIX kernels) still. That doesn't mean there are certain places the only choice is the Linux kernel but that alone isn't enough to make it a monoculture in this sense of nearing domination of the space.

Browsers are the most oft cited example of monoculture issues be it the old days with IE or the new days with Chrome, I'm surprised you haven't run across comments on that one over the years.