The ability to make it proprietary is in fact the reason why Apple didn't adopt gcc and became the main supporter of clang/llvm instead. So the issue is already here. Fortunately Apple's ambitions only center around keeping full control of the Apple walled garden (the apple main building is a wonderful symbol of it). And should a strong proprietary competitor outside of Apple's ecosystem emerge, people can always fork clang and put it under GPLv3 or similar.