Democrats have been blowing off the actual left for as long as I've been allowed to vote (before the election, we're a small subset of losers who "can't compromise" so no one should try to give in to any of our demands but then after the elections when they lose, it's all third parties fault because we didn't shut the fuck up and get in line like they did). They've continued to move right ever since then.
Maybe the USA has moved rightwards. If the Republicans have moved right more than the Democrats have moved left, arguably that moves the centre of gravity right as well.
Maybe they're right that we're not a big enough group to cater to but I 100% feel like there are a lot of voters like me but they just don't bother to vote because they see no point in it. A LOT of my positions are pretty fucking popular even across the aisle like M4A and not funding a genocide but Democrats would rather go after Republicans instead of at least trying to do some of the shit us dirty lefties want from our politicians. I'll be sad for America if this proves to kill Harris but I sure as fuck won't' be sad that The Democrats still haven't learned their 2016 lessons of "Not being Trump" isn't enough this go around to get people to throw away their principles.
Maybe, but that's a big unknown. I suspect a lot of people don't bother to vote because they're not bothered voting. Some are profoundly apolitical. Also there are people happy to expound on politics, often passionately, but who don't vote. They don't do anything else either: campaign (even non-partisan), or get involved in any local / national issues, or in fact do a single thing to work towards political change. Maybe they'll occasionally help out a fundraiser for the local hospital. But "Nothing ever changes so why should I?" sure sounds a lot better as justification for inactivity than "I want to complain but can't be bothered doing something about it".
Sure, you can find a lot of people who agree with you on certain issues, but it's the aggregate that matters. Every position taken wins some voters and loses others, and you have to get the right blend of policies and ideology to tie up enough supporters. In bulk, not supporting Israel would appear to be a flat vote loser in the USA because its population is more pro-Israel than pro-Palestine by a substantial margin. There are people like yourself outraged by support for Israel to the point it's a deal-breaker. However, there are all the opposite people for whom not supporting Israel is a deal-breaker (some of which are also likely to be major targets for Democrats), plus all the people for whom supporting Israel is a one of a basket of pros / cons that might move them one way or the other.
You might find 70% of the country wants socialised healthcare. You might also find that half of those voters will never, ever support your party because they hate LGBTQ+ rights, immigrants, feminists, etc. and it doesn't matter how much you promise them "Hey guys, socialised healthcare: you want it, we'll really do it!". They'll still retort "Fuck you, American-hating f*****-lovers" and vote for the other guys. So are you willing to then compromise on those other things they hate? Tough ask. And what voters might you lose to do so?
This will be even more complicated by the amount of money swilling around politics, particularly from the rich. As they are so critical for funding, they also call a lot of shots to the detriment of general public will. But the cost of opposing them is that they kill your party and its policies. You can promise socialised healthcare and even if 70% of Americans would vote for that in theory, in practice you might be under 50% (a lot under 50%) once all their think tanks, media outlets, and paid politicians have flooded the airwaves and savaged the crap out it.
As disappointing as the Democrats are, they have to work with the circumstances and system that they're in. If they were wiped clean, whatever replaced them would be in exactly the same system, with the same voters and voter preferences, same billionaires, and end up pretty much the same. It might cause a significant reordering of the policy basket that a notional "Neo-Democrat" party ran on - for instance it could be more economically left but less socially liberal - but the inevitable end result would be that it still would have to make shitty decisions that would alienate voters it would theoretically want. Proportional representation could help deal with this and provide an outlet from stifling binary politics in a parliament, but the president is still going to be a winner-takes-all heap of grubby compromise.