You know the parties used to just pick someone and told the people "This is who you get to vote for!".
It's pretty clear that a strong majority of the primary voters wanted a change. You can criticize that Democrats let it go this long and that Biden was allowed to run for re-election in the first place, that is certainly warranted. But ultimately, they can pick their nominee any way they want. There is no legal obligation to go with the primary voters. Although in other circumstances, like what they almost did in 2008, will likely piss off the voting base and blow up the nominee's campaign before it starts. Not so much this time and with Biden endorsing Harris, most of them will likely accept Harris as the nominee without too many issues. Or they won't and there will be a convention fight. But as far as getting her or someone else on the ballots? There shouldn't be any issues there. At least until the corrupt SCOTUS decides to invent new laws again.
