Corbynism is working exactly as intended
Corbyn and McDonnell want the Labour Party destroyed as a parliamentary force because their faith tells them the cause of revolution requires it. They believe that if things get really bad the people will rise up so it is worth losing everything gained since The People’s Budget let alone since 1945. Their faith inspires them to let everything burn and be happy when they splash the accelerants around.
This is why they are so keen on Brexit. Firstly, leaving the EU will cause terrible harm to the UK economy producing exactly the worsening in conditions of life they fervently believe are a necessary step towards revolution. Secondly, committing Labour to actively supporting the government splits the left and reduces the electoral position of the party bringing about the collapse of Labour as a parliamentary force.
They are of course wrong in every regard but they and their acolytes in the media are united by their faith rather than politics. This is why they want to capitulate to right wing populism rather than fight it.