Kamala brings nothing. California is not in play. There is nothing gained.
It's a matter of what does Biden need more to win the election. If he chooses an African American woman as his running mate as he is being pressured to do, does he have enough support in the midwest?
Kamala Harris or Stacey Abrams won't help him win the midwest. They will more than likely ensure he wins the African American vote.
If he picks say Klobuchar, Whitmer, or Baldwin to help him win the midwest, will he still get the African American vote? He was not popular among African American voters until he was endorsed by African American leaders in the Carolina primary. Before then, he lost that part of the vote.
If he doesn't pick an African American woman as his running mate, will this piss off the African American's. Make them feel disenfranchised? Enough to vote for Trump or elsewhere?
If he decides to choose someone to help him with the midwest, Klobuchar seems the best to do that. It is unclear whether biden would have won any primaries in the midwest had Klobuchar not dropped out and endorsed him. He was not doing well there in polling.
Whitmer or Baldwin would likely net him their home states, but would they help him win the midwest?
Biden needs the midwest to win. It is why Hillary lost. Hillary won the African American vote. So Biden probably would if he decided to go with a mid west helper.
Elizabeth Warren is a popular candidate that would run well with Biden, but she doesn't help him in the midwest. She brings the more progressive voters back to the table. Will they come anyway?