I think you are underestimating the resolve among Democratic party leaders to keep Trump away from the White House. It isn't just political, or a power grab. They fear as many of the citizenry here do that he is a fundamental threat to democracy. Unification in the party seems at least in part based on that fundamental tenet. It's as much a guarantee as there could have been that they'd fall lockstep behind her quickly.
In addition, though I am not familiar with the rules, I assume the Biden/Harris money is tappable by her and not others. Getting a campaign going from scratch would have been near impossible for challengers. In fact, I was loud in my insistence that it would be difficult even for Harris and she has proven me dead wrong so far.
As I noted before, if she was well-served by her staff and party leadership they would have been planning for this contingency anyhow given Biden's age, long before the debate. While the circumstances and rapidity of the decline in support for Biden may have taken everyone by surprise, she should have been ready to step in at any time regardless -- that's simply common sense and appropriate planning.