You make reasonable points, but here are a couple light rebuttals which ultimately amount to my idea that the risks of replacing Biden outweigh the risks of keeping him on the ticket.
I'm skeptical of the candidate even having to set up a full campaign, because they'll have the name recognition already. If it's the result of 5 debates between younger Democrats including Harris, that is going to be blasted on every news channel. Not to mention, a giant campaign isn't exactly helping Biden. The negative press alone from them trying to shield him probably set him back enough that he'll lose.
Regarding the ads having already been reserved, I won't pretend to know how any of that works, but wouldn't you think that would also hinder Trump? As all his ads would be toward Biden and not the candidate? He would somehow have to pivot his attacks. Harris is easier for Trump to attack, because he's been doing it for years now, along with Fox News. Regardless, I don't find much sway in that argument , just because I'm not sure ads are really going to do much to swing moderate or independent voters.
I don't think the public is politically aware enough to realize Harris is the "next in line," as you infer, so I really don't think they'd be alienating black or female voters.
(Side note: I think the whole idea of Harris being the one to be "passed over" is such a bullshit modern political take. Just like how it was Hillary's turn, or Biden's turn. That kinda shit is what got us here now, instead of choosing the best candidate we always seem to choose the one who should be, or who is due next. Really fucking frustrating.)
Finally, the media is already having a feeding frenzy on Biden, a well-tested candidate with lots of experience. He is down in the polls in swing states, he's getting grilled by his own party, and his numerous attempts of damage control have done little to assuage the concerns from congresspeople, but I'd also guess voters, as well. I'm sorry but I just don't think Biden can beat Trump, and I think he should step aside for either Harris or, in my preference, let the DNC shake it out with some debates.