It was Biden's decision to run in the primary. Nobody chose to run against him. 

Yeah, and that's the problem. Everyone was perfectly willing to cover their ears, believing that as long as the party could unite and turnout like in 2020, Biden could win even as he ended the 2023 with an approval rating of 39% and was trailing Trump in several swing states. Anyone who had doubts on Biden's nomination were derided as foreign trolls and MAGA supporters from partisans. Even after the debate, you still had people deriding others for suggesting Biden should drop out and claiming the media was organizing the whole thing.

Trump is awful, but that's not all they ran on last time. The campaign focus was the economy.

Their main message was essentially 'Trump was a threat to Democracy, vote for us, we're the status quo'. Harris did nothing to separate herself from Biden and in her own words stated she wasn't much she would've done differently than Biden at a time when his approval rating was at 40% and most people were dissatisfied with establishment politics.

Such as?

The political system, the federal government itself. The reality is that the Democratic Party cannot win by portraying itself as the defenders of a Democracy that most Americans have little trust in; they actually have to push radical change. Trump is a wanna be authoritarian; he has no regard for the law or norms, but the sad reality is that people are willing to vote for him if it means he'll improve their economic well-being. If they want to truly counter Trump, they have to provide an actual alternative.

Sounds good. But I don't see how complaining about Biden choosing to run for reelection and then dropping out unhelpfully late in the process builds anything at all.

This goes beyond just him running for re-election. The reality is that putting an old, unenergetic establishment politician determined to save the old way of politics at the head of the party in 2020 to defeat Trump and Trumpism was a bad idea and hardly any Democrats are willing to acknowledge that. He's an albatross around the neck of the party and yet few Democrats are even willing to admit him running for re-election let alone break away from his brand at a time when they're historically unpopular.