Support for rejoin can be as high as it wants to be. The bottom line is that Sir Keir has ruled it out. It's not happening. He doesn't want to lose Labour Red Wall votes.
I think people have to be realistic about these things. If they want to rejoin, vote for a party that will take us back in or at least are receptive of the idea of applying. That's not Labour yet people want to vote for them. Pick your priorities.
As far as I am concerned, the next GE will be the last time I vote Labour unless they announce PR as part of their manifesto. I can understand Starmer dodging on this next innings, but failing adding it to the next GE after would just mean we are on a circle jerk. I still remember Blair said he would move to PR when he was in office.