So, speaking as a Democratic voter who is somewhat politically active, though I live in a heavily Democratic area (DC suburbs; Trump is a threat to a lot of people's jobs).
A lot of people were not convinced that Biden has dementia rather than just being old and tired. I have a child who stutters and several friends who are speech therapists and we all agreed that some of his issues were disfluencies rather than dementia. People don't know how to spot those unless they are familiar with them. The media also ran the story into the ground to the point of making many people angry at them instead of Biden. I know at least a dozen people who cancelled their New York Times subscriptions over it, and other outlets too.
There's no process for picking an alternative that would be any fairer. Americans don't want party insiders picking the candidate; it's not how it's been done here for a long time. Everyone in the Democratic Party is haunted by the contested convention of 1968 and how that election turned out, so no one wants it to go to that. Harris has a better claim to popular legitimacy than anyone else, since she at least got votes for being Vice-President. And say what you want about identity politics, but passing over a specific Black/Asian woman would look terrible.
Biden won in 2020 not because people loved him so much, but because people were tired of Trump and Covid mismanagement, and some of those people have amnesia now. A big part of winning is actually making sure the base actually votes in key states, not just chasing a small and elusive pool of swing voters. Kamala is absolutely animating Black women, who are the Democrats' most reliable voters. She's an Alpha Kappa Alpha from Howard and the entire Divine Nine have announced an effort to support her campaign. There was a lot of tiredness about Biden going around.
The Republicans got caught off guard by the announcement. Yesterday they were resorting to making fun of Harris for liking Venn diagrams.
Platform? Honestly we know roughly what it will be. Similar to Biden.