The campaign is not about Harris (or Biden, or anyone else) vs Trump. Almost nobody is thinking "Should I vote for Trump or not?". Trump's vote has been locked in since 2020.
The point is to get the large numbers of Americans who either rarely vote, or who sometimes vote but couldn't bring themselves to vote for an even older white guy than Trump, to turn out and vote against Trump.
Given that, Harris has some advantages. She is a woman (yes yes, she mentioned her pronouns that time on stage, we'll have to get over that), she is not white, and she is a competent speaker. In fact her professional background is quite similar to Keir Starmer's, as she was the equivalent of the DPP in California. I don't know if further head-to-head debates are planned, but Harris will take Trump to the cleaners if they happen.
For what it's worth, I don't think Harris would be a particularly good President. But that's not the choice right now. The choice is between a moderately competent individual who will appoint a selection of reasonably qualified advisers, versus a narcissistic psychopath who will jeopardise the freedom of the whole of Europe, on top of the damage that he will do to democracy in the US.
It's going to be difficult, but without Biden it's at least possible. The Democratic Party also needs to have competent (meaning, as a minimum, able to get people's names right) national leadership in place to oppose what Trump and co may try to put in place.