I wasn't thinking about that aspect, but you are certainly correct about that. I was more thinking about what you mentioned with "no campaign infrastructure", etc.

Yep. That's part of the problem with thinking that a Gretchen Whitmer or Josh Shapiro can come into the race at this late date and run effectively. Those guys don't have a national campaign team put together that can run a national race, much less field offices in Nevada or Arizona. Staffing that up takes time. The advantage of a Presidential primary (for a non-incumbent) is that the race for the nomination gives them many months, nearly a year in some cases, to build up campaign teams in most states around the country.

Harris' advantage is that she's already on the Biden ticket - so many (most?) of the people in the campaign right now have already worked with and for her already to some extent, and many (most?) would be willing to stay on the campaign if she moved spots on the ticket. Not all - but the expectation would be that the Biden-Harris campaign organization would mostly continue in its current form, just with Harris switching in for Biden.

That doesn't work as well - or at all - if there's a six week "shadow" primary among different Democratic rivals jockeying ahead of the DNC convention. The current campaign would basically stop, and whoever won the contest would end up formulating their own national campaign organization from mostly scratch - with only three months to the election.