Would I welcome a significant primary challenger to Biden right now?
Only if it's obviously someone who has the juice, the electability, overwhelming popularity and a level of confidence and competence that is absolutely undeniable, even to Joe.

It would have to be someone who is so damn good that even Joe might consider volunteering to pass the torch.
But I have little to no worries that Joe can win the election.
I would entertain shuffles in the Veep slot but it's not as if I think Kamala is terrible, she's just been more of a behind the scenes Veep, which plenty of Veeps down through history HAVE been before her.

She is not a Dick Cheney. But gimme someone who is absolutely incredible like I talked about above and....sure okay, let's consider that option. They COULD BE a take charge Veep who reinforces Joe and is prepared in the event Joe DOES have to step aside. But that IS the historic role OF ALL Veeps.
LBJ got the job handed to him under worse circumstances, but let's remember, the government and the party were prepared for that possibility all along.

So it's not as if it's much different. Joe Biden doesn't have a wiped camshaft or a catastrophic connecting rod failure in the offing.

"Oh, Joe's a little old and a little tired, let's go ahead and give it to Trump!"
Are you ****ing kidding me? ????