As Sir Winston Churchill said, those who fail to learn from history are doomed to repeat it.
If you have even a cursory look at the Labour past, you have -
Michael Foot - unelectable - far left and out of touch and completely unapealing
Tony Blair - centre right - hugely successful - at first
Ed Milliband - unelectable - completely unappealing
Jeremy Corbyn unelectable - far left and out of touch and completely unappealing
Keir Starmer - slightly more centerist, out of touch, unappealing.
The only success was Blair, who understood the aspirational attitudes of modern Labour voters and exploited it to huge success.
Corbyn, as I pointed out on another thread, has about as much idea of what modern Labour voters want as I have about the dark side of the moon - and paid the price for living in a fantasy past that never existed, and being utterly out of touch with anything north of the M25.
Now Labour want to repeat that failure by appointing another Corbyn who is cut from the same cloth, has the same Marxist fantasies, and will lose the next election for the same reasons they lost the last one.
Now I am not a politician, but i can work that out for myself simply by looking at what has happened, and understanding why.
The powers-that-be in the Labour Party have a vested interest in simply not seeing what has happened and why, because it goes against their core beliefs, however whimsical and far-fetched those ideas are.
Politics is about pragmatism - ignoring what you'd like, and dealing with what you've got.
Corbyn was singularly unable to embrace that simple concept, and there is no doubt that McDonnell will be similarly blind and deaf.