If Biden were to drop out, the dominant bloc of delegates might well be his own, who would instantly become free agents. They may or may not choose to support the “winner” of the rump primaries, if there is one. I doubt they’ll go rushing to runner-up Bernie Sanders. More likely they’d lift someone near Biden’s ideological wheelhouse to the nomination, even if it meant deadlocking the convention on the first ballot and then joining forces with newly liberated superdelegates on a second.
Pareene doesn’t directly discuss the alternative scenario of Biden actually losing the nomination in the remaining primaries, but that’s probably even more far-fetched, particularly considering the deal his campaign just struck with Sanders’s, which will give him reallocated statewide Sanders pledged-delegate slots in exchange for placing Sanders supporters in them for purposes of representation on convention committees. Biden is much closer to the nomination than ever, and even if he loses the remaining primaries badly, proportional representation will make it very easy for him to grab the brass ring.