There's a myriad of interesting OSes which run a Linux kernel. Containers, for one (such as Docker). Qubes, for another. Third, Tails. Fourth, NixOS. And so on, and so forth. These 4 examples might each run a Linux kernel, but they each have very different design goals and aims.

I'm much more concerned about the lack of diversity in hardware space... specifically, the lack of open hardware.