It takes little effort on Apples or Googles or Microsofts part to take advantage of an LLVM dominated world to close off their own changes to it and try to force developers to use their own proprietary LLVM distributions on their own operating systems. It stops every other company in the future from taking advantage of all the great LLVM tech to implement their own CPUs in terms of LLVM IR, so they never need to publish their ISA and they can lock down their platform with a blob LLVM of their own. The only thing stopping any of that from happening is GCC being a still competitive alternative.
If LLVM came to dominate the compiler scene to such a degree that GCC were irrelevant, it opens the flood gates to any major company forking LLVM into a proprietary paid for compiler that they require on their OS to profit from. It lets you do anything from what Apple is already doing with Swift, where you create a programming language with a proprietary compiler, or you could go from the other end and implement a proprietary translation unit so that you never need to publish your ISA (something Nvidia never does) but that allows all the LLVM compilers to target it.
Right now, nobody can close off their LLVM contributions because a hobbled and fractured LLVM ecosystem is one that cannot overcome GCC. In the same way Apple and Google had to cooperate on webkit until it was so dominate they were in a position to fork and do their own things with it once Gecko was rendered effectively irrelevant, but the difference is that webkit was LGPL and Clang is its own permissive license - they still cannot directly modify the free software parts without redistribution, so it is harder to make a proprietary webkit, but Apple surely has succeeded since they have their own proprietary patchset on top of trunk webkit nowadays.