Gore in 2000 was popular enough with the general electorate in the years leading up to his launching his campaign. And really, so was Clinton leading up to her 2016 campaign. This is just one pollster, but Gallup's polling of her listed here:
https://news.gallup.com/poll/1618/favorability-people-news.aspx shows her only dropping into negative favorability #s in July 2015, by which point it was pretty late for a new primary challenger to jump in, and the Democratic establishment had already committed to her.
So one question about Harris's ability to clear the field is how are her favorability #s among the general electorate faring a year from now, or two years from now? Because I have to imagine that if Biden to declined to run again, and Harris stepped into the frontrunner role, we'd see a ton of second guessing among Dem. partisans on the "Can she really win?" question if her poll numbers with the general electorate aren't doing so well.
