Thinking that these kids would be solving meaningful societal problems if they were not working on HFT is delusional at best. To be able to tackle hard problems and make any meaningful impact you have to be very passionate about the problem that you are researching/trying-to-solve, and no amount of money is going to make this passion materialize when it's just not some of it inside you already. Now don't get me wrong I'm sure these people are really smart and I would be even willing to bet that the distribution of IQ at Jane Street is tighter than many top research labs even when the latter group produces more tech and accolades than the former; raw intelligence is not the only ingredient of world changing minds.

At the end of the day I believe that if CS/finance was not "cool" and paid the way it does (specially at the level of HFT) most of those kids currently there would go back to the good old law/medicine...