1. The idea that "They can beat Trump" is the single most important consideration is a losing strategy for Dems. Yes, it will get them them the Dem votes but that's about it and that would be the case no matter who they nominate. To win over enough Independents and possible Republicans to actually win you have to have a better message than that. As far as I can see, the only other message from the Democratic party consists of raising taxes on everyone and everything, no borders,"free" everything and granting amnesty to illegal aliens.
Just don't see it...
There are more Republican Governors and Senators under 50 than Democratic, so the Republican party does have a bench. It's not going to be people who typical Democratic voters like but that's a different story.
There was a major problem for Democrats that they discouraged anyone from running against Hillary Clinton in 2016, which meant that when Booker & Klobuchar decided to run for the 2020 nomination, they would be overshadowed by newer figures like Kamala Harris (elected to the Senate in 2016), Beto O'Rourke (came close for the Senate in 2018) and Pete Buttigieg (legally too young to be President in 2016.)
A complication is that Democrats had done poorly in statewide and congressional elections in 2010 and 2014, which meant the bench was much more limited.
There was an argument that Democrats really failed to prepare a next generation of leadership under the Obama administration, although that's complicated by all the former Obama staffers who just elected to Congress in 2018.
Sincerely,
Thomas Mets