This has been the Dem plan all along. Many have predicted this. While it's anyone's guess who they will pick (my guess is Newsom), anyone other than Harris is going to gain a ton of Dem support. They intentionally destroyed Biden, with media support, at the last minute to temporarily tank Dem hopes.
This will result in a huge amount of new Dem energy when they get a fresh face. And it will make it less obvious when they cheat (no one would believe Biden won, but no one is going to question extra votes for someone new).
We would have been better off if Biden stayed in. It was a guaranteed loss. I have zero confidence that Trump will win now.