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.

Linux powers most of the internet, most mobile devices, and most embedded and IoT devices. Maybe it's not a complete monopoly, but it's close.

Except it doesn't, go check termux's future on Android, as its developers refuse to adopt the Java APIs that are actually what it public on Android (Linux kernel syscalls aren't a public API).

https://developer.android.com/ndk/guides/stable_apis