Keep in mind that in the 'Trusting Trust' example, the compiler has to be smart enough to realize that you are building another compiler (and only then insert the backdoor). I can imagine that a back-doored GCC would recognize when you are building another GCC, but it would be hard for an old version of GCC to recognize eg a modern LLVM or even GHC, I'd say.

Similar, the lower down you go, the harder it is to put that kinds of smarts in.

That might be one reason to stop at this point? (Not sure.)