May be the AI had existed in the Galaxy and halted by some internal reasons, but leaved after it some self-replicating remnants, which are only partly intelligent and so unable to fall in the same trap. Their behavior would look absurd to us, and that is why we can't find them.
Adding additional unneeded assumptions does not make hypothesis more likely. Just halting and not leaving any retarded children explains observations just as well if not better.
For a moment lets assume there is some alien intelligent life on our galaxy which is older than us and that it have succeeded in creating super-intelligent self-modifying AI.
Then what set of values and/or goals it is plausible for it to have, given our current observations (I.e. that there is no evidence of it`s existence)?
Some examples:
It values non-interference with nature (some kind of hippie AI)
It values camouflage/stealth for it own defense/security purposes.
It just cares about exterminating their creators and nothing else.
Other thoughts?