Supply-chain backdoors are one thing, but the saving grace about the backdoors injected by trusting-trust attacks like in the OP is that they're quite fragile.

Supply-chain backdoors are one thing, but the saving grace about the backdoors injected by trusting-trust attacks like in the OP is that they're quite fragile. If the shape of the expected target program changes sufficiently, then the backdoor will fail to propagate. And propagating the backdoor into programs that have yet to be written requires an effectively omnisicient adversary. As long as you have multiple implementations of a toolchain, you can repeatedly use them to compile each other in every possible configuration (and then use the outputs of those compilation steps as inputs to the same process, repeatedly) and then compare that the outputs are bit-for-bit identical until you reach a point where you're assured that either none of the compilers are backdoored or all of them would have to be. So yes, they're scary, but there are countermeasures.