That's an interesting thought, but San Francisco aside, it seems like most people moving out of cities are just moving to the suburbs of those cities, which shouldn't affect the presidential election calculus?
I know people that have moved, but forgot to register. Its a fact of life that voter registration will take the back seat when managing more complicated things in life - like a move
That seems plausible. Still, I think the "exodus from cities" theory doesn't apply much to American cities outside of New York and San Francisco. Places like Philadelphia may have even seen a small uptick from NY emigration. But this is just a guess, it would be nice to see any statistics (I haven't found any).
Here in San Diego, for instance, we've generally had net inbound migration. It's pretty much the opposite of Philly, though, in terms of both coronavirus and violence. But it is an "alternate choice" backwater city, compared to SF or LA, kind of like the way Philly is to New York.
I wonder if that is true for people who were also likely to vote? I know registering to vote was one of the first things I do whenever I move, and I've also voted in every election since I've been allowed to.