> Who rob banks? Robbers. (i.e. I don't think 'Robbers robs banks' is right.)

“Robbers robs banks” is certainly wrong (it should be “Robbers rob banks”), but so is “Who rob banks?” (it should be “Who robs banks?”).

As a follow-up to my sibling comment https://news.ycombinator.com/item?id=18410528: if your point is that the question should parallel the answer, note that the exchange could also go ”Who robs banks?” ”John Dillinger robs banks.” A question that can only be asked if you already know its answer (so as to decide whether the verb in the question should match a singular or plural noun) is not so useful!