I was around and adult for the SDP phenomenon. I even voted Labour that once. Shows there is no real appetite for centreist sell outs in my opinion.
I disagree about 2018. Some of my family have been involved with Labour and the unions as long as I can remember, from an ethical perspective. The lack of integrity of the Leadership had always been a problem, as has career opportunism at all levels.
However the 'extreme left' who joined during Jeremy Corbyn's time, if they were extreme, were not necessarily members of groups. I was tempted, but I didn't, being a member of such a group at the time, but I took an active interest, because the Labour party has always claimed to be the voice of people like us. Two friends of mine did join, and were repelled by the local party machinations. We all knew of these tricks, having been campaigning on land and community issues against Labour councils which had few or no scruples. We only managed to save community assets by raising the game and campaigning in the community on the ground, and got some appreciation for that from the nicer Labour people.
So we rather hoped to see an energised Labour party at a local level and that new members would be welcomed and respected. I'm willing to accept that there was a 'Militant Tendency' style contingent who messed things up, but I saw no evidence of it. Where is the evidence?