I mean, there are valid reasons to be anti-Harris, she's still a right-winger. Leftists aren't going to be happy voting for any right-winger by default.
Harris could take a firm anti-genocide stance, or promise something big and popular like Medicare for All, which would see increased voter turnout from the Left. However, given her track record, I doubt she would pivot in this direction, instead focusing on being a younger and healthier Joe Biden, essentially. That should make liberals more likely to turn out, so still a net-gain for the Dems.
It's really too early to tell.