I was watching Shirley Williams on TV last night (she wasn't talking about all this, she was on the Somme commemorations programme), and I thought exactly the same, remembering the SDP.
When the SDP mob split off several friends tried to persuade me to join SDP, but my feeling then was that Labour centrists could beat militant tendency and the like, and still be an electable left of centre party.
This time round I don't think that's the case, I'll be supporting a breakaway. A new party might be small but at least it would be eligible for coalition. I doubt anyone would co-operate with a Corbynysta rump Labour, it would be doomed to perpetual opposition.