Countering the trusting trust attack

You misunderstand. I'm not saying 'if you prove that this compiler is old
then you will invariably detect the Thompson hack' I'm saying 'if it is
likely that this compiler is old then your chances of detecting the
Thompson hack go way up'.

(And the Thompson hack *was* specifically relating to quined attacks on
compilers and other code generators. Viruses are a much larger field, with
Thompson hacks as a small subset. It is possible they are converging, but
I see little sign of it: attacking compilers isn't profitable because
they're relatively uncommon on the man in the street's machine.)