If the title seems weird, remember that Corbyn only got onto the ballot in 2015 because some MPs felt it would be good for Labour party democracy if the left was represented. The current rule is that 15% of MPs and MEPs have to nominate you if you want to stand as Labour leader. That rule can preclude anyone from the left getting onto the ballot given the current composition of MPs and MEPs. Corbyn’s team want to turn 15% into 5%, but the great majority of MPs will do what they can to stop that happening.
It is this that is keeping Corbyn in power. A majority of Labour party members want to be able to vote for someone from the left in any future leadership election. If the 15% stands, that seems almost impossible. These members therefore want him to stay in power for as long as it takes to change the 15% rule. But Labour MPs have no intention of giving way on this, because they believe that if the rule is changed the left will have a stranglehold on the leadership, given the current composition and views of members. That is how Labour MPs are keeping Corbyn in power.
Corbyn’s popularity among the membership has changed significantly since the 2016 election. Then he won easily. Since then his popularity has decreased substantially, in part because of his poor handling of Brexit and also because it has become more difficult to claim that his unpopularity in the polls is due to Labour disunity. But MPs will not put up a challenger, because they suspect the challenger will fail. It will fail because by voting for someone who supports the 15% rule members believe they will be voting for their own disenfranchisement. This has become a power struggle between MPs and members, and Corbyn is becoming just a pawn in this game.
If that seems fanciful to you, just consider what happened last time round. In an emotional response to the referendum defeat, MPs passed a vote of no confidence in Corbyn. It was a challenge that was far too soon, giving the impression among members that Corbyn was only doing badly because Labour MPs were out to get him. The challenger that MPs chose for the subsequent election contest, Owen Smith, adopted virtually the same policy proposals as Corbyn/McDonnell, but crucially did not back changing the 15% rule. As a result, the members understandably responded by saying if you do not trust us, we do not trust you.
Owen Jones has called for a deal whereby Corbyn would step aside in return for someone from the left being on the ballot to replace him, but I cannot see MPs agreeing to that. Their strategy is to exhaust Corbyn and the membership, and preserve the 15% rule at all cost.
This is not so much a battle about policies as a battle about power. MPs intend to win it by keeping the 15% rule, and waiting for Corbyn to go through sheer exhaustion, or waiting for members to give up and hand power back to MPs. Labour members hope Corbyn can hang on long enough to change the 15% rule. Events like Brexit, Scottish independence and a general election just pass by, only mattering if they have implications for who achieves final victory in this war.
Now many in the political commentariat will assume that of course Labour members cannot be trusted, and so the strategy of Labour MPs is correct. Their pleas for Corbyn to ‘for pity’s sake go’ are just for show. But if that is also your view, consider two points.
The first point concerns MPs. The current decline of the Labour party as an effective opposition did not begin with Jeremy Corbyn. It began, as Larry Elliott clearly delineates, with the global financial crisis that happened on Labour’s watch. The unforced errors began when Ed Miliband, and the team around him, made the fatal mistake of not challenging the Tory narrative about the previous Labour government. That mistake meant that Labour was blamed for austerity, Labour were not trusted with the economy, and Miliband’s poll ratings just got steadily worse, even though many voters were experiencing an unprecedented decline in real earnings.
It was MPs reaction to the 2015 defeat, and a general belief among many of them that Labour had to move further to the right, which ensured a victory for the left in the subsequent leadership elections. Corbyn and McDonnell tried to create an opposition of all the talents (or at least those that were willing) and reach a consensus on policy. But at each turn they were met by a small group of MPs that constantly briefed against them, and other MPs that did nothing to stop this. Anyone with a clear head could see this strategy by rebel MPs was totally unproductive: Corbyn had to be seen to fail on his own account.
The most recent misjudgement by MPs was over Brexit. Again Corbyn gets all the attention, but it was the majority of MPs that decided they should focus on the challenge from UKIP and vote to give May total authority over the negotiations. I think this misjudgement epitomised almost a decade of bad decisions all of which involved an element of appeasement. John Curtice has explained why, with most Labour voters choosing Remain, voting through Article 50 was a very odd decision. As I’ve argued here, the referendum placed no obligation on MPs to vote to allow May to choose a Hard Brexit. If you are unconvinced about this, imagine the referendum had been to do nothing about climate change. Would MPs have been obliged in that case to follow the ‘will of the people’? Would it have been right to ignore a clear consensus among experts that their implementation of the vote would be disastrous? The actual EU referendum has the same ingredients as this imaginary case.
The second point is about membership. There is one clear Labour achievement since 2015, a huge increase in membership which numbers more than all the other parties combined. Is it really healthy that this should be regarded by most MPs as a problem? Arguments that half a million people are either Trots or under the influence of Trots are nonsense. Although it was Corbyn who inspired the increase in membership, it was not the left that wrote the rules that allowed the leader to be chosen by the membership alone. Having tried to make the leadership election more democratic, it is not plausible to then turn round and say you got the wrong half million.
There is a legitimate concern in allowing members to have almost complete control over which MP they choose as leader. It is not that they will choose a leader who is too left wing, but instead that they will underrate the importance of being able to win general elections, or at least not fully appreciate what it takes to win these elections. To this I would add that the membership may be insufficiently ruthless in getting rid of a leader who is failing to win, particularly when this failure is often very unfair (given our media, for example). But with questions like this we have also to ask whether MPs would be much better at judging success and punishing failure, and the experience with Miliband plus the constant attempts to appease the right suggest not. 
Elliott describes how many of the policies that Labour currently put forward - Brexit aside - are broadly popular when tested with the public. However if you attach the name Labour to these proposals, their popularity decreases sharply. Those who say Labour have become unpopular because they have ‘moved too far to the left’ misunderstand what is going on, or have more questionable motives. Elliot describes it as Labour becoming a toxic brand. Corbyn is part of that, but only part. The record indicates MPs are also responsible for the current mess.
As John Curtice explains, the question is not will the brand survive, but how and when it will recover. I can see two ways forward. The first way is for MPs to trust their membership, change the voting rules, and allow Corbyn to resign sooner rather than later. If he does not resign, the left has to bring forward a challenger. With good judgement, the new leader can lead from the left but with policies that the broad church that is Labour can live with, combined with a strategy for convincing the electorate that would make Owen Jones happy. The second way is that MPs keep the 15% rule, wait for Corbyn and the membership to give up exhausted, and then hope for a Macron type figure to emerge from among their number. The key question is which of those two do you think is more likely to give us a plausible candidate for Prime Minister?
 The Conservative Party shows that members can learn. They chose Cameron not because they wanted to modernise, or be green, or hug hoodies, but because (after 2 defeats) they realised the party needed to bury the image of the nasty party and they saw in Cameron a person who could achieve that.