If Labour had held to its 2017 position - we will honour the result, we will seek to negotiate the best possible agreement that protects workers rights, environmental standards etc - then the fury at the sense of betrayal would not have manifested itself. That sense of betrayal and a resurgent Tory Party under Johnston simply repeating 'get Brexit Done' was an entirely predictable and devastating combination. So predictable that many of us did predict it. Yes Corbyn was a massive issue on the doorstep but it was unquestionably Brexit that generated the really and deep fury
Had Labour stuck to its position - and let's be clear that was Corbyn's preference - then in remain areas it would have then been a case of explaining why democratic decisions must be respected, why a LD vote is always a wasted vote and why Labour would safeguard lots of the things that remainers held dear.
We will never know how it would be turned out. But we do know a) how the alternative played out and b) who was at the centre of moving Labour to the mess that was its final position on the matter.