I strongly suspect it wasn’t the manifesto that lost. Sadly Corbyn turned out to be a clown, just not up to the job. We also need to factor Farage/Banks buying ‘red wall’ votes just to tip the seats Tory. That was a very real factor, the seat down the road from me fell as a result of that, as did many others. There is no doubt Corbyn caused a lot of damage to Labour as by being so feckless he inadvertently re-empowered the right of the party, who a remarkably toxic force and will not give that land-grab back. Another reason I feel the Labour Party is finished. It had an attempt at returning to core function, but totally blew it.
Agree with the JC comments, particularly re: the damage which is still under-recognised, but what is the party's core function?