As far as you're concerned, Biden dropping out would be an unmitigated disaster no matter who were to replace him, but that's a different topic than I am contemplating here.
I have not said I want Biden to be replaced—or even that I don't want that, for that matter. The only thing I want for sure is for Trump to lose, and despite the thirty or forty or whatever it is percent of total polls that are flooding the zone out there showing that Biden is tied or ahead or at least holding ground versus last week, I am not confident that Biden is the guy who has the best chance to do so. He might be, but I truly have no sense of it either way.
What I am saying is that if a change were to be made, I do believe that the party's best chance to beat Trump this fall would be Gretchen Whitmer. I'm not interested in discussing the weeds about how she would build a team, or get donors on her side, or any of that, because I am talking in terms of "if", not "I want". If I were coming from "I want", I would have to contemplate it. Instead, all I am saying here is, if Biden were to be replaced, then based on articles I have seen that have discussed various possible replacements, I believe Gretchen Whitmer would have the best chance of beating Trump in a general election.
(That's setting aside Michelle Obama, of course, who, while polls I have seen show that Democrats believe she would be the runaway favorite among all options to beat Trump in a general, she is not a serious option in any way to replace Biden.)