This was a candidate problem not a party one! Clinton lost because of her association to failed trade deals, she let Trump run to left of her on many issues. Nobody here thinks Biden would have lost had he ran.
What about Congress, state legislatures, governorship? This isn't just the presidency. But if we want to talk about the presidency, maybe the Dems should step away from establishment elitist politicians like Hillary Clinton and nominate someone who is charismatic, populist, and authentic. If the Dems can turn around and nominate someone completely different as quickly as the Reps nominated Trump right after Romney, then they shouldn't have a long term problem. But that will take a party change both at an insider level and with the voters as well.