Has it occurred to people like Jamie Reed that while Corbyn may be rubbish, that is what his party wants. If he doesn't like it then maybe he is in the wrong party. Strangely enough he had reached the conclusion that the parliamentary Labour Party is out of step with its voters because Corbyn has poisoned them. It's actually not true. Most Labour Party members prefer Corbyn to the people he is most like.
Party activists are voters but the vast majority of Labour supporters and voters are neither party activists nor members.
The stupid £3-membership rule is what caused the problem. I know of several self-proclaimed Conservative supporters who claim to have voted for Corbyn in order to make Labour unelectable.
The MPs represent their constituencies, and realise that Corbyn can only lead their party to electoral oblivion.