This is some serious Stockholm syndrome to convince yourself that bypassing any primary for the voters to select the candidate, and instead just install someone who nobody would have picked is something an awesome move. Why do you suppose they had a presidential debate in June, like 3 months before they are every other time?
No Trump supporter actually cares either way cause they weren't voting for either of them anyway and Harris is probably even less popular and an even worse speaker than demented Joe Biden. It would have been a decent guess that this might be the straw that broke the camel's back to get you guys to finally realize that the party you support flat out undeniably has 0 respect for you or "our democracy", and kind of shockingly, the reaction from all of you instead is "well why should we have the right to pick our candidate instead of donors and party leaders".
No, the VP succeeds the president during the term if he has to leave office during the term, which you aren't getting either. She isn't taking over the presidency for the term, Biden is apparently staying till the end of the term as of now, and she is ONLY taking over as candidate, for which the process is always that the VP can run their own campaign with everybody else to become the next candidate.
Republican party leadership and donors were absolutely against Trump becoming the candidate both in 2015 and even this year, but voters overwhelmingly told them to GTFO with the Jeb Bush or Nikki Haley horse shit, and forced them to get behind the candidate the voters wanted. Your party is the party of institutional power who truly despite the electorate and make you bend to their will rather than them bending to yours.