Not everyone who didn't vote for Harris voted for Trump. There were other candidates available - including another woman who will likely once again get part of the blame as she did when Hilary Clinton lost - and many chose not to vote. It's been acknowledged since this election season started that small parties and voter refusal was going to take more from Democrats than it would Republicans, that Democrats needed to strongly focus on outside of their voter base to the growing number of independents - and they didn't. They kept preaching to the choir and their wealthy backers (see Harris at her expensive fundraiser during the civilian-led post-flood efforts in Appalachia because federal help was no where to be seen and states other than Florida were really struggling - and then days into it Harris insulted DeSantis for doing the work without talking to her, as if he's meant to do that, which Biden did not back her on at all thankfully). Their 2020 tactics of focus on the evils of Trump wasn't going to work a second time, too much time has passed and too much shit has happened since.
Part of the issue the Democrats have had for over a decade now is that the their strongholds on the West Coast and parts of the North East have found it acceptable to dismiss, insult, and shit on pretty much everyone in other areas. That on top of decades of promises that they'll ensure the equality everyone is so sure Harris was going to bring (Dems have been promising to codify abortion since at least the 90s, probably earlier, they dangle these every time, but mostly only inflate their own power) has resulted in fewer swing battleground states as people feel the Democrats no longer can represent them.
Democrats will largely blame the voters and some of them will blame small parties, it's easy to do, even though they'd been warned for well over a year the landscape they were going into and really only have themselves to blame. Eventually they'll have to look at themselves or fall apart. It'll be similar for Labour - they got in mostly cause people were sick of the Tories, they won't be able to rely on that next time. We'll see if they can learn.
This feels a lot like Brexit conversations - point out Remain's campaign was absolutely shite, and you get told that as the status quo they didn't need to campaign (or at least I've been told that when discussing it) and people are just dumb. That attitude has never gotten us anywhere, it certainly doesn't suggest that said person cares about others and equality, but it somehow sticks around.