Biden was lowkey one of the most progressive Dems in recent presidential history, more than Obama, yet managed still push bipartisan policies through Congress. Progressives like AOC and Bernie stayed loyal to him for a reason. Non-maga GOP heads want Trump gone.... if he loses this election, the GOP is free.
So, if Kamala's adds a moderate Dem to her ticket, she could potentially get those non-maga, right leaning votes. The Progressives lose, like most of the time, because Kamala herself isn't a progressive but the Dems when the election.