The reality is that Democrats could’ve run someone else, and Trump would still have a good chance of winning the presidency. Every nominee of a major party starts with 47% or so of the vote irrespective of the opponent, fundraising, debates, etc. Adds to that the Electoral College that gives an advantage to the GOP, and you get a situation where the Republican nominee can always win no matter how many deficiencies he has.
And so while the Democrats can do better, the voters have the final say. Too many Americans are okay with this, and no Democrat candidate can sway them. As long as this is the reality, expect people like Trump to have a chance at winning.
One more point: Democrats won 5 of the 8 most recent elections, and two of their losses were by tiny margins (2000 and 2016). In 2022 they defied the odds in the midterm elections. And so, they’re not that bad.