If Corbyn is barred from a leadership vote by raising the required number of nominations

(which would also bar anyone else from the left from competing), the outcome of the contest could not be considered legitimate. No one elected in a contest in which the left are blocked from having a candidate(in a party where the overwhelming majority of ordinary party members are on the left) could fairly claim to be the validly elected leader of the party.

You'd end up with someone only the MPs approved of...which means you'd end up with someone who stands for nothing.

There are no huge numbers of voters out there who want to see that happen.