... what will be the Democrats' strategy for 2024?
What do you even mean by "strategy for 2024"? The party typically doesn't collectively think strategically like that. Whoever wins, wins.
Don't you think the party will consider why two Democratic tickets - both considered very capable, and one even very popular - lost against a very unpopular, controversial and incapable Republican and think about what they could do better?
What I mean is that it's a reach to suggest that the parties think strategically like that. Even when they "anoint" someone, like the Dem. elites did for Clinton in 2016, it happens as a consequence of polling. Party elites rallied around her because they thought she was going to win the nomination anyway. Maybe they'll rally around someone new in 2024 if they lose in 2020, but it'll most likely again just be a consequence of following whatever the polls at the time say will happen, as opposed to some well thought out strategy.
The strategy is more likely to come from the candidates themselves. If Biden blows a winnable election, then the candidates in the next cycle will try to avoid whatever mistakes he's thought to have made. But that's different from using strategy to pick who to nominate in the first place.
