I think the one scenario that could work for Brejoin is if Labour rebranded itself as the rejoin party to stave off the threat from the Greens.

The Remain contingent is lost anyways. The other side of the electorate is turned off by their waffling. This would give them a cause to rally around and allow them to consolidate their electorate again.

With enough Brexit voters now either dead (they skewed old!) or having changed their mind, plus younger folks that weren't eligible to vote back then being very pro-EU, that might just do it.