If someone other than Harris were VP, I'd say the Democrats are moving against Biden to force him to step down. However, having Harris assume the presidency hurts the Dems more than it helps them going into 2024. I think the plan is to keep Biden in the White House, but damage him to the point where even he realizes that it's futile to run for reelection. They will then quietly back their chosen candidate in the primaries against Harris.
Rest assured that if they boot Biden before 2024 they will not put Kami in charge.
This is a very long runway to simply undermine a Biden 2024 run.