By your logic, the left need to dig in and sabotage as much as they possibly can up to the next election. Try and contribute to Starmer losing the next election and then you'll determine that proves they were right all along.
No. Most of those who were undermining Corbyn from 2015-17 were chastened by the 2017 result and thought 'perhaps we were wrong, perhaps he can get us elected' and halted the campaign, to concentrate on May and Johnson and Brexit.
Corbyn was given his rope and after two more years of seeing his lack of leadership and confusing messages, the country decided to hang him with it.
What the far left need to understand is the country isn't hard left so they shouldn't try to impose or sneak their brand of Labour on it. Not only is that undemocratic, it doesn't work.
What the far left need to understand is a Labour government is far better than a Conservative one.
What the far left need to understand is when the country rejects them that is what we get.
Labour never wins when it is divided. Look at the history.
Ramsey MacDonald managed to get elected PM, but then the left decided he wasn't left enough and couldn't countenance working with other parties in a National Government during the Great Depression. Labour went from 288 seats (to the Tories 260) minority government (with the support of Lloyd Georges liberals) to MacDonald winning the largest ever mandate from a British PM with 554 seats. Unfortunately 473 of those were Conservatives, 68 Liberals and just 13 National Labour. The Labour Party under Arthur Henderson won just 52 seats.
Then we had the post Atlee period when Labour were riven between the Gaitskillites and the Bevanites and were out of power from 1951 until Wilson brought everyone together again with a more centrist compromise. Wilson was elected PM twice.
Then, when Labour went down the Michael Foot route on the left, the country rejected them. In fact Kinnock had to spend years fighting to oust militant to get the party electable again. He narrowly failed in 1992, largely because of Major's personal centrist appeal to the nation.
Step in Blair. Three times centrist Blair was elected and he did a hell of a lot more for workers, the NHS and Education than Thatcher or Cameron, May and Johnson since.
So the message to the far left is you can't win, but you can certainly do a job for the Tories you supposedly despise. Can't you just be smart?