Even if clang was a "first-class citizen" compiler for the kernel and was picked up by some big distros, there's an absolutely gigantic market of companies designing and building embedded systems around GCC. There already is a LLVM/Clang monoculture in some areas (e.g. research, "modern" C++ shops), but while I don't have the numbers or anything, I'd imagine that those fields make up a very small percentage of worldwide C/C++ developers. A lot of those boring embedded systems companies have low margins and don't think particularly highly of software developers, so they'd never consider refactoring all of their tooling and projects to use clang unless they absolutely had to.
If the kernel _dropped_ support for GCC, you might see an actual monoculture develop. But I don't see the kernel _supporting_ GCC, or particular distros choosing to build their kernel with it, making any real dent in the huge market share (for lack of a better term) that GCC has built up. It might seem that way if you read HN and talk to people working in SV, but GCC is still pretty ubiquitous.