I was a defender of Corbyn ever since his election but even I am now weary of trying to argue his case.
I think he may be viewed as a stage Labour had to go through, to demonstrate the total disconnection between the PLP and the membership. I also still think he is more sinned against than sinning - he's been consistently attacked and undermined for the last two years, by the media and his own party. The refusal to support his leadership and the coordinated resignations were unforgivable.
Unfortunately, we can't just blame them for it. He was not the man for the job. Given the opportunity, he has failed to do anything with it. In part he has been hampered by the lack of cooperation and backstabbing (he must have a huge back, there are so many knives planted in it) but his own weakness, and the weakness of his team has not helped. As remarked below, the three line whip fiasco was a completely foreseeable, avoidable and utterly unnecessary disaster. A conscience vote would have kept a lot of people onside, avoided the charge of hypocrisy and not affected the outcome of the vote. Arguing the media would portray it as weakness or 'ignoring the will of the British people' (what happened to the 48% - have they been stripped of their citizenship?) is to overlook that whatever he did would be portrayed negatively by the media.
So fairness, consistency and avoiding unnecessary damage should have been the consideration. Instead, all three were sacrificed and the result is a disaster. Nothing good has come of it, and a lot of bad has. After facing front bench rebellions against a three line whip - even some of his whips voed against the party line - Corbyn realises he can't really do anything other than issue a 'written warning' to the rebels, asking them not to do it again?
And it was all utterly predictable and foreseeable. Anyone could have seen this coming. Except, it seems, Corbyn and McDonnell.
Maybe someone should reactivate the old 'Corbyn Warnings' twitter account ... only this time, to warn Corbyn of the bleedingly obvious consequences of some of his more mutton-headed decisions.