Corbyn and his like have been trying to take over the party since the 80s. Apart from Attlee's post war government (arguably), the Labour Party has only ever been trusted by the electorate when it has presented itself as a 'safe', centrist party. If the majority of the PLP split to form a centrist party that your traditional labour voter and your middle class conscience voters can identify with, then it will completely marginalise the hard left and the 'Labour' party will become nothing more than a protest movement full of students, unionists and Ernest, which will remain (as it is now) completely unelectable.
It's not about being elected, it's about getting rid of those modernists that have stolen your favourite table in the corner at the back of the pub. Once these newbies have finished their pints of craft beer and moved on, the old'uns can claim it back and sit there watching everyone else, collectively moaning about how the pub isn't as good as it used to be.