Labour were also moving to the left before Corbyn, Blair->Brown->Miliband was a progression away from New Labour back to the unions. Corbyn is just completing the transition back to the original Labour movement. Not an ideology I share, but at least it's true to the founding ideals of the party and it's union roots.
If there was any sense in politics, Labour would now split, with 'new Labour' supporters forming their own progressive centrist party. They'd probably mop up the last few Lib Dems and maybe one or two Tories.
It won't happen though, as the 2-party mindset of British politics (a lot of which is due to the mechanics of FPTP) discourages smaller parties and instead favours 'broad churches'. As a result, we get the ridiculous position of Ken Clarke and John Redwood being in the same party, and likewise Jeremy Corbyn sharing a party with Liz Kendall.
How long before we recognize that this position is untenable? IMHO a broader spectrum of parties, working together to find a majority consensus on issues would deliver far better government than a monolithic duopoly ruled by fear of the three-line whip. It's why I feel electoral reform is needed and hopefully Labour are now coming round to the same position - one they would then share with the SNP, Lid Dems, UKIP, and Greens - who together would represent a 60% majority of voters (if not seats).