"the best laid plans of mice and men".
Here is my analysis of what went wrong with the Dem bid.
Harris was originally put on to broaden the diversity of the ticket in 2020 to gain votes. Biden was happy with this, as she probably did that without being a challenger to him. We all assume that politicians within a party are united, but they are always looking for threats from within. The idea at that point was that Biden would never run for a second term, so Harris' suitability for president wasn't really a key issue. Biden would quit at the end of term 1, and a new ticket comprised of a new presidential and new candidate would come in for 2024.
During Bidens first term, Harris became a convenient dumping ground for policy that might reflect badly on the big man. She was given a series of hospital passes, including handling immigration and the negativity against her piled up. However, it was (in theory) a non issue because she was going to be dumped at the end of term 1, and Bidens performance would be the one more closely assessed by voters on Democratic candidates for president in 2024.
Roll forward a few years, and Joe changes his mind. He's going to run for a second term. Harris is still there and probably can't easily be gotten rid of, but she doesn't challenge his power, still diversifies and ticket and fills the requirement for having all the rubbish dumped in her direction.
This was all going swimmingly until Biden gave up on his second term. At that point it was too late to have any other candidate other than Harris because of the way the funding works. She then came in, not only with a questionable ability to do the job, but with the huge albatross round her neck of all the rubbish the Biden administration had saddled her with.
Now we are where we are.
The TLDR of all this to me is that yes, VP picks are important and not always that easy to brush aside. There is very much a likelyhood that a VP pick may somehow get parachuted into the main job when you are dealing with men as old as Trump and Biden.