Harris actually has a good background story, 4 years ago she wasn't ready to run for President and ended her campaign early, but that's normal, one way to set yourself up in the future are abortive runs that get you name recognition and experience with debates and talking to crowds. After 4 years as VP, she looks far more poised.
As a prosecutor, she avoids the "Progressive soft on crime" label, as VP she obviously has the experience, and like Obama she fits the "black, but not too black" mold, father is Jamacian and PhD in Economics at Stanford, mother is Indian PhD and expert on disease - so both immigrants but both elite. She'll appeal to women, younger voters, black vote and South Asian vote without alienating white voters. Give her a white male VP in his 50s (Kelly, Cooper) and it's a balanced ticket. The Jewish husband allows her to be credible as both pro-Israel but less lenient than Biden, who is stuck in a time warp.
We'll see if she's learned to campaign, but if she locks up the nomination going into the convention, she'll avoid the animosity of a primary campaign (and the soundbites that can be used against her). Right now the Democrats have taken the media attention from the Republicans and this should continue for weeks up to the convention.
The biggest problem for Trump is the age issue has gone from an asset to a liability - he's now the oldest person to ever run for President and the media will start to scrutinize every meandering moment (sharks and batteries, Hannibal Lector and so on) as possible evidence of pre-Alzheimers (father developed in his early 80s and Trump is a prime candidate, obese, doesn't exercise, high stress, family history).
With Biden out of the race, he'll garner elder statesmen status, his administration will get the "rosy glow," and Trump attacking him will alienate people.
As far as Mike Johnson, anyone who has to cut off a speech b/c the teleprompter broke shouldn't be trolling on the basis of mental incapacity.