Just my 2 cents worth...
First cent: I agree that the sentence for the engineer involved probably won't make him feel any worse, and he will probably be the most cautious mechanic in the world now (assuming he gets to be a mechanic again), but can we be absolutely sure? Will we bet more lives on it? From what I know of this case (very little, I admit), I can't say 100% yes or no on either question, so I think we must err on the safe side.
Second cent: Like most jail sentences, the intent here is not only to punish and/or reform the jailed person, but to also dissuade others. From the volume of discussion here, that part seems to be working.