My first thought when Disney made this announcement about Black Widow was that it was at least partially a tactic to get better deals with the chains that boycotted Disney. It certainly would have some of the theaters worried to lose the blockbuster movies to Disney+. A game of chicken (sort of like what some rumors said about the Sony-Marvel deal with Sony that unfolded earlier).Technically, you can. Already, the changes from "theatrical only" to switching to hybrid basically is the end of "you can't do that." So they could easily say, "well, we changed it already, we're changing it again."
Also, though this particular scenario I'm about to suggest is unlikely, if all the chains decided to boycott Disney and refuse to play either Cruella or Black Widow, that would make Disney immediately reconsider and make them theatrical only.
It does make me think, BTW, about the time that the Cinerama Dome had to back out of a plan to premiere The Hateful Eight in order to get The Force Awakens. Would it have been better, in some ways, if Cinerama had said, "Well, go and take it, then. I'm keeping my promise to Quentin?"