If I had to bet, I'd give slightly better than even odds that the decision for Biden to withdraw has already been made (or is very close to being made) and that the delay in announcing it is because the Dem power brokers don't want to say so until they have worked out the logistics of what happens next.
The Dems are in disarray. It wouldn't look good to cave to popular demand to move past Biden without a solid plan in place.
Whether that's a plan to go all in on Harris or open the convention to some degree to pick a new slate, beats me.
A defiant Biden also argued that he had been made the nominee in the Democratic primary process and that to step aside would disenfranchise those voters, though no prominent Democrats chose to challenge him in the primaries. “How can we stand for Democracy in our nation, if we ignore it in our own party,” he wrote. “I cannot do that. I will not do that.”