What's up with Kennedy's recent polling though?
This is just a hunch, but...
I suspect that a majority of his voters were swing voters who leaned democrat but were worried about Biden's age. With Kamala Harris being much younger, a lot of those people are no longer concerned about the capability of the democratic candidate's ability to hold office and are now back to voting for the democrat.
I'm just going to add on that I never understood all the concerns about Biden's age. He was beating Trump with a decent margin in the Midwest, which was the easiest route for him to win. Not to mention that, even if Biden died in office (fun fact: since the risk of death plateaus at 50% once you turn ~75, Biden and Trump had the same odds of dying before they finished their terms), what was wrong with Kamala Harris replacing him?