I think Harris is too close to Biden to wash the smell off. She'll take all of his staff and advisers and run the same game. Also the press will eat her alive with all the talk of "did you undermine the President", "are you comfortable replacing your running mate". The election will come to a screeching halt, democrats will have no ability to get their message out.
Biden stepping down because of COVID is the golden goose of getting out of the race. You couldn't ask for a better excuse.
In terms of replacements, I get the impression America is sick of really old people looking like fools running their country. Whilst i'd always tend to a progressive running, I look at the options and basically if I was going "anti-biden" it would have to be buttigieg, whitmer or newsom. Younger, good speakers, charisma, sharp. Not my personal choices, but they are the establishment "anti-biden" group.
I don't know whom the progressives could really offer up to challenge those 3. It would have to be someone very young, sharp and not going to spend the entire election talking about socialism and NATO the whole time.