Meanwhile, all the other platforms for which LLVM would be the dominant platform and for which there aren't extant major backers are basically on the hook for whatever way LLVM wants to play it, including extinguishing support for their platform.
No, really, I mean it. Explain in detail how FreeBSD will become utterly unusable if LLVM went closed-source today and deleted any FreeBSD related code from their now-internal repository. Or AMD's OpenCL implementation. Or any other project using LLVM.
Sure, it's technically possible to start up a fork tomorrow. But, realistically, it's also just as possible to recreate MS Windows from scratch.
That comparison is utterly silly. You already have the entire source code for LLVM, and forking it is simply pushing it to another repository. That's the complete opposite of Windows, where you'd have to recreate it from scratch.
Well, either way, you're setting yourself up for the risk of the source going closed and you having to support development yourself.
Well yes, but all that does is put you in the same spot as if you had picked a closed-source component to begin with.
And don't think you're safe with GPL. Any GPL code can be re-licensed (and thus made closed-source) if the copyright holders agree. Of course, exactly as with BSD, the code already distributed under GPL/BSD will stay GPL/BSD.