It wasn't a scheme to slip Harris in from the beginning, but it was a scheme.
A majority of Dem leaders knew Biden was too old, in decline, and would not be up for the job of a 2nd term. But they decided to lie about it, misrepresent his condition and ability to the public, and chance that it might work out or, worse, figured he'd lose but it wasn't worth the risk to their own careers to tell the truth. That is, they put their own standing in the party and their own political careers over their country. Indeed, if some of their rhetoric is to be believed, they risked a dictator taking power and ending democracy in order to save their own intra-party reputation.