There'll be a move back to the centre and away from populist politics, yes. But this is an epochal crisis, and that wont hold.
New parties are required on the left. A mass based party with an organic link to the working class through unions and community organisations is possible.
Starmer can keep his trainset. I dont want it.
Yes the debate on the left is now very much between those who want to change the system, and those who want to put the system back together (for fear that any change can only come from the right).
The left are essentially lurching between irrelevency and hegemony. It needs to find a way to be counter hegemonic.
We have needed such an organisation you outline above for decades. A big part of the problem, is that very very few on the left of Labour are willing to countenence a break with Labour of any kind. Thats before you get to the Unions, where the old syndacalism of "we do industrial matters, Labour does political matters" still holds.
Up your way you are fortunate that much of these alliances could be challenged and ultimately broken via the national question. It's not as easy in England and I'm not sure how it looks to begin such a process. In many ways Corbynism reflected an opportunity to do such, but it was squandered.
What always amazes me, when you speak to influential people high up in the left in the LP (and at times some senior figures, before they became famous) is how they refuse to budge on the idea that Labour isn't the sole answer. The right of the party know they have this devotion to and terror of leaving on the left and in a lot of cases are just far more ruthless and smarter at leadership level, so push the left of the party to ever ridiculous situations. I mean they openly campaigned against Labour over the last 4 years, and I'm not aware of a single expulsion.
When we need a serious conversation, we get Owen Jones and Paul Mason telling us "don't give up" "it's still our party" "they're only doing this becuase they are scared" "any talk of something to the left of Labour makes you a Stalinist" "he will start to adopt Corbyns policies soon" etc. The purge of the left will continue, until people stand up and say enough is enough. And it is going to get a lot worse over the coming months and years. They will not let Corbyn be the national treasure Tony Benn was allowed to be.