People think the C compiler community is dominated by GCC and Clang, and it is, but there are literally 1000s of implementations out there in the wild. Most are necessary, because we need code generated for some obscure processor architecture that's completely proprietary, but you can create that "backend" in LLVM itself - it's a new target architecture instead of e.g x86.
The great thing about LLVM is that it's effectively the quickest (and probably the best) way to generate machine code without putting in too much effort, for a language. Whether that language be a research language or an existing industry language (say, C), that kind of establishment is hugely valuable.
A great example of a good monoculture is the Go monoculture. Sure, there's gccgo, but the proportion of people using that vs. the reference implementation is minimal, and that reduced fragmentation is actually a good thing for practitioners (which most engineers are, not PL researchers).