A lot of democrats could have won this election. Ultimately the big mistakes were allowing Biden to run unchallenged, then sticking with Biden until it was too late. Harris then had an impossible task to win.
If the democrats had an actual democratic process, and put their best possible candidate forward they may have won. Instead this election was very much a repeat of 2016 - the wrong candidate, being favoured through to the election by the DNC. In 2016 the DNC closed ranks around Clinton because of fear of Bernie and also because of a crazy notion that it was "her turn". Biden didn't run when he should have. This time Biden ran when he shouldn't have, and other strong candidates in the party didn't get a chance.
But it was more than the candidate - the election focus was totally wrong. 1/3 of the electorate did not vote - and this election is not a story of Trump breaking through. Trump got 74m votes in 2020 and about 74m now. The Dems got 81m votes in 2020 and 71m votes now - Trump is basically static; but the Dems lots 10m votes because they ran a bad campaign. Those missing 10m voters are in the 1/3 who are not included in polls; because Trump has not broken much above his 74m ceiling. The Dems floor fell out under them instead.
The polls always showed 50:50 but that was just "likely voters". Really 1/3 support dems, 1/3 support reps and 1/3 weren't going to vote. That vast pool of people are not all never voters; the missing 10m are in there. THAT is where the Dems should have been going for votes. Forget the republicans; they should have been reaching out to the disinterested and disenfranchised. A positive message that actually addresses their concerns.
The "moderate" Republican votes were never in play nor worth courting, and the abortion and democracy focuses were not the priorities of voters. The dems needed to listen to the actual voters - and the message of what the voters cared about is clear: the economy. The Dems needed to have a clearer message on the economy - "it's doing great" does not tally with voters experiences who are living with high cost of living after inflation. Prices haven't fallen back, they've just stopped rising as fast. The message to voters should have been "we've done some stuff but there is more to do" and offer clear policies are wage growth, housing/rent costs etc. Give the disinterested in particular something to vote for.
So yes, maybe Bernie would have won. But lets not forget he chose to endorse Biden, not run in the democratic party primary. So it's actually his fault too.
Only Dean Philips, Marianne Williamson and Jason Palmer actually stood up and challenged Biden in the primaries, and they were criticised for doing so as if they were the reason Trump would win.