Although Corbyn himself is (understandably) loath to admit it, there is a good deal of similarity between his own rise and Donald Trump's (and, probably to a lesser extent, Bernie Sanders, another old white man who somehow magically appealed to young voters). In particular, the bit about taking on the internal party machine and winning so convincingly is a parallel people would do well to remember.
It's (almost certainly) far enough away from the next election that Corbyn can recover from the party disunity and broaden his appeal. I doubt it very much -- but the mockers had better not be complacent, especially if (as is very possible) May, Davis, Johnson and Fox make a complete mess of Brexit. Where do you think the angry voters would turn then? Not to UKIP, at least not in large enough numbers.
It's fun to point and laugh now, but the joke may yet be on his detractors. In particular I would enjoy a Corbyn victory in 2020 if it meant seeing TTT's smug "I joined for £3 and voted for Corbyn to screw Labour over" grin wiped off his face.
At any rate, now that the Parliamentary Labour Party basically has to shut up or try an SDP thing all over again, Corbyn has time on his side.