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.
This! In spades as we say on NSC.