The Democrats have a structural problem.
The party is defined as being composed of the people who are already elected. So the priority of the Democratic Party ends up reflecting the priorities of those who are already in office, which is to make sure the incumbents get reelected.
This means there's very little incentive to expand the electorate (which would mean younger voters, who are likely to vote younger candidates, so that threatens the aging incumbents), or spend resources in expanding the map (because by definition there are no incumbents there whose interests are represented in the party).
For as advanced as the US political system is, it's incredibly backwards when it comes to professionalization of the political parties. A good comparison is the BJP in India. Setting aside policy, ideological issues for a moment, what they're really good at is being professional. The head of the party is not elected, and constantly rotates the party representative in each election, keeping their bench deep. They also have a soft age limit.
In a way, Donald Trump's greatest contribution to the Republican Party was destroying the incumbency advantage for Republicans. As a result the Republican slate was completely refreshed with younger (although generally worse) candidates, but while it may have made the party significantly worse from a policy/ideology perspective, it has made it politically stronger.