It's possible that message was lost, but honestly, I don't know if there's much we can learn from the tactical errors of the Harris campaign other than "don't be a member of the political party in power during a time of inflation."
The upside for Dems is that Harris was a mediocre candidate, and her political career ending isn't that bad. It would've been a bigger punch had they nominated Josh Shapiro only for him to inevitably get pummeled.
They called in their C-list candidate late in a ball game which was already over before they realized it. Their A-list candidate can be nominated next time.
Much like how DeSantis used his political opportunity to run in the primary, candidates have a narrow window to strike and be successful. Because of his tactical error, that torch has now been given to JD Vance.