The truth is that it rarely matters; most voters have made up their minds long before such announcements are made, and even seemingly commonsensical gambits like choosing senators or governors from crucial swing states rarely has a discernible impact on the outcome of elections.
This was certainly true in 2016. (Find me the voter who said, “You know, I was pretty sold on this Trump character until the former secretary of state he called ‘Crooked Hillary’ chose a Playmobil caricature of a business-friendly centrist Dem from a formerly purple state to solidify the ticket. Changed my mind in an instant.”) It will almost certainly be the case in 2020, when perhaps the single most divisive incumbent president in American history faces a former vice president. Voters have had more than enough time to make up their minds about these people. No one who was prepared to vote for Biden will re-evaluate because he chose a rising liberal senator instead of a fellow veteran of the Obama administration or vice versa.
This is not to suggest that Biden’s campaign could choose anybody. But the choices that might actually make a difference — an ally of Bernie Sanders or even the man himself, a political amateur like Abrams — are outside the realm of plausibility. Biden is not going to select a principled progressive or anyone else who would be unacceptable to his party’s establishment in the top job (his age makes this an important consideration).