If people switch from GCC to Clang/LLVM in enough numbers that Apple think they can get away with it, Apple will, in a heartbeat, close the development of Clang/LLVM and make all new versions proprietary.
You understand Apple aren't the only developers of Clang right? It's their project, but there's nothing stopping anyone else (say Google, which are also huge contributors) from forking their own version of Apple decide to do something that would surely hurt them more than help them.