More than anything, not having a primary cost Harris. The primary is when the party has to court the left more, we find out which ideas/candidates have enough momentum to be on the platform, then discussion opens up to the broader population in the general. Biden should have stepped back sooner.
Without a primary, the platform wasn't battle tested, and the left had no leverage over the campaign. I dislike HRC for many reasons, but her 2016 general campaign was stronger and further to the left than it would have been if she hadn't spent a season sparring with Bernie on the campaign trail.