it may be necessary for May to win in order for the far Right to be utterly discredited during her tenure. If Corbyn were to win, both Labour and the Tories would revile him and block anything he wants to do, and work to discredit the Left, which would just prolong the life of the far Right.
Corbyn is way to the left of the modern Labour party; like Sanders and the Democrats in the US. The Center Left parties have wholesale abandoned their traditional constituencies, and it is going to take a lot for them to revert to their previous stances in any genuine way.