Personally, I would have have gone for a more progressive candidate (Warren, Bass). I am not only saying that because I'm biased as a social democrat , but it is purely a strategic consideration. If you want to have widespread influence in national politics, it's important to find a balance in how you want to attract as many different strides of the society by choosing the right president + VP, but also the right platform.
For one thing, I agree with Biden's decision that the VP should be a woman and POC if there's a suitable person who fits these criteria. It makes the party more representative. It has the psychological factor that most people will be more susceptible in voting for a party that has politicians who look like them, talk like them, etc. These voters are considered low information seekers, which is perhaps the most important electorate (=large portion electorate) to have in trying to win an election. To make it clear, low information seekers is a political science term for voters who are drawn to issues as appearance, media presence, charisma, etc. anyway, not policy based per se.
Another consideration is attracting voters who also value policy (high information seeker). These are the kind of voters who try to reflect, analyse and compare information from different media sources, political programmes, etc. It is a substantial part of the electorate that you also need to consider. In a winner takes it all contest like the American presidential elections, it is important to have a platform that gets as many voters from different strides as possible in order to win. A big tent organisation like the Democratic Party represents many political strides and thus has the capacity to attract a wide audience.
However, during election time, it is crucial that the party sends the message to people that they represent their ideals. In order to do that, they need a platform that is balanced between a wide appeal (representing the interests of many people, like working class, middle class, POC, etc.) & ideologically consistency (sending a clear message that all Democratic representatives want to make healthcare accessible to all, for example). Finding this balance is crucial, but it cannot be done if you have political leadership that represents only one group (centrist New Democrats) within a big tent organisation like the Dems. If you choose someone like Kamala Harris, who in my opinion shares almost the same view as JB, you risk alienating a big portion of the party (1/3 caucus Dems and potentially 1/6 electorate).
This winner takes it all mentality will only maintain the division in us vs. them (establishment) and will not serve the party in the long term (e.g. risk of voters going to the Greens, form own party or more conflict in the party between progressives and centrist in deciding direction of party). This already happened in the Obama/Clinton pro corporate centrist era and it helped pave the way to a Trump administration. Let's hope the Dems have learned their lesson.