Devastating, but not entirely surprising. First and foremost the people to blame and point fingers at are those voting for Trump, they are the ones who decided to support the insanity.
There are a number of things democrats could have handled better. I think their campaign has for the most part been good, and all data I have seen suggests Kamala was as strong a candidate as anybody. She united the party in record time, came off as likeable and caring. Sure she might have had some wacky views in 2019-2020, but did anyone really believe they weren't just pandering? What the campaign failed to do was deliver something for male, especially young male voters. This has been a headache for the left in the west for a decade now, that the feminists and the so-called 'woke' have not managed to find a strong motivating message for male voters.
Besides that, I think the campaign was strong. Hence Harris' major problem lies in the governing of Biden. The handling of the border has frankly been a disaster, they started the term appealing to the far left on this issue, then way too late they realized this is a major issue for the average American. The fentanyl crisis and Afghanistan withdrawals were problematic too, but I doubt it made a big difference in the election outcome. The economy probably did make a difference, even though Biden didn't really do any significant mistakes here, the inflation would have gone up no matter what he had done.
Anyway all this combined could definitely explain a Harris loss, but to a standard republican, not a radicalized MAGA lunatic. Had she and Biden done everything right, Trump would still be looking at 45 % of the votes.
So there is the main problem - the MAGA movement is fundamentally a dishonest political movement, not motivated by any real political ideas or visions, but feelings of anger, fear, paranoia and pride. It is a movement that is not actually explaining honestly what their issues are and what they believe would solve them (if they even agree on any of that), hence it is absolutely impossible to address with rational political solutions.
There are no real solutions to that, which are possible within the American system.
Europe is on a similar trajectory, but at least here the path is not set in stone. Politicians need to focus on solving some of the driving factors: increase manufacturing jobs, shut down the option for spontaneous assylum to avoid huge and fast demographic changes, facilitate the distribution of wealth, heavily regulate social media platforms, or even better- abolish them completely. Same with the use of artificial intelligence. Europe should also invest in military to defend itself, and unfortunately this includes getting substantially more nuclear weapons. This all seems very dark, but unfortunately it is not about making the World a better place anymore, but saving Europe from fascism, which is all we can hope for now.