Your carrot/stick hypothesis is spot on.
Biden is not going to improve, and time is short. They must do something, and they’ll do it energetically.
Trouble is they have no good hitters on the bench. Clinton, Kerry, those are the unpalatable vets. California Boy will not beat Trump, either.
I think the decision must be made before the convention, every day adds more obstacles.
So it has to be Harris. There will be much agonizing over the right shade of lipstick on the porker, but she is the legitimate successor by law and they do need to preserve some appearance of legitimacy.
Watch the market. If there are tremors, they will move even faster to resolve the problem.
If they can blackmail Jill than likely they can do the same to Harris. It's a tough call but they may scare both women literally to death!
The CIA basically runs the show. They have it all wired,or so they think. If they want to get rid of someone generally they can make a case to do so. Or, they farm the wet work out and let someone else take the heat.
Either way they usually get what they want.