Is not the lesson to learn from Labour in the 1980s and early 1990s that it not only needs to exclude the ultra left, but then needs a progressive policy approach that reflects modern Britain and rejects the dogmas of the past?
You talk of not learning from the 1980s. The idea that the 1980s tells Labour anything other than that associating with the far left is electoral poison is nuts.
The dogmas of the past include the Blairite Third Way. It's far too similar to what Johnson is doing now to be distinctive if it was recycled in term of the big government and private sector pump priming. Slogans and warm words which just sound like recycled Blairism - progressive, modern etc - and have almost nothing to say about current challenges eg policies for a Britain outside the EU, are clearly not making an impact. The Tories seem far more enthusiastic than Labour voters, not that they are intending to vote for Labour, but it confirms Thatchers claim about her achievement and their ability to maintain hegemony.
Unless Labour can develop a distinctive programme which can hold its voters from 2019 and gain some more, mostly from those who didn't vote last time as direct switching from the Tories is unlikely, they haven't much chance of turning things around. Polls suggest they aren't likely to do it.
It's all part of the Chesterfield Conference* "let people who actively oppose the Labour party into the Labour party" tradition. See also affiliation attempts by the Communist party in the 1930s.
Michael Crick's book on Militant made the point that Labour Party people with the "no enemies on the left" approach are typically very naive and do not realise (until too late) that the groups they let in will not be allies in the fight with the Labour right, but will purge them from their positions just as happily.
A reminder that the CDU "no enemies on the right" meant the suppression and elimination of them!
"People may say what they like about the decay of Christianity; the religious system that produced green Chartreuse can never really die."