Man, there is just so much at play. Now we have the Israel conflict, which is turning some young voters against Biden. Abortion carries weight w/ voters, clearly - but voters can get distracted easily. Shiny new issues will come up in the next year, and we have no idea if they will help or hurt. Like you said, some knew what would happen w/ SCOTUS in '16, and it wasn't enough.
And you're right about Harris. She is simply unpopular, and that hurts the ticket.
Dems feel confidence that Biden can beat Trump, because he has. But at that time, he was the challenger, running against a divisive leader who was the worst jobs President in modern history. Now Trump can be the challenger, and take Biden to task on a wide variety of issues. Perhaps not honestly, but no one can spin like Trump, and he'll make Biden seem like the worst Prez in history by the time the campaign is done.
It's gonna be such an ugly year. It will just be less ugly if Haley is the candidate for the GOP.