Dozens and dozens of UNIX kernels (with Fuchsia being the exception I believe..? Not sure). Yes I know kernels have far outgrown POSIX, but it's still a form of monoculture. Diversity in all dimensions matters imo.
Unix kernels could use more cohesion, not less, in terms of API. Even if that API is implemented mostly in userspace
View the kernel API like HTML. Browser diversity is good, but they need to be implementing the same interface if the smaller players want to support any hardware that works with Linux