The Democratic opinion of the situation wasn't uninformed. It's universally known that Trump has a core base that's incapable of being eroded, but that base has objectively shrunk from 2016 due to time passing and COVID. It was universally understood that he polled poorly with younger audiences, and his base did indeed shrink by 2020. It was understood that it would continue to do so for 2024.
Polling as well has become very fuzzy since the rather catastrophic misses in 2018-2022 so they were watched but not taken as gospel. Given that, it wasn't unreasonable to assuming that they had a very high probability of winning as of a couple weeks ago.
Regardless of that, Democrats did not slack. They didn't hold course with what was subjectively a poor candidate, they pivoted when they needed to. They took the run seriously, pulled out the big names, including a bunch of former republicans.
Sometimes you can do everything right and still lose, that's really okay. I do think effort should be put into the various reasons why exactly non-core voters went for Trump. Like, you're not going to win the incels who think Trump is going to make women ownership legal, but find those weirdos who got confused because Biden wasn't on the ticket and talk to them. Find the ones that voted on inflation despite having no idea what it was and talk to them, figure out what they actually want. Hit the streets, get some data points, spend 2 years crafting a message, test it on midterms, and refine it for another 2.
... Assuming we get another chance in 2/4 of course.