The thing is, even if they don't drop XB1 now they will have to eventually. Not long after release I'd wager, maybe by end of 2022. If Infinite is supposed to be a 10 year platform with constantly adding new campaigns and whatnot, they have to stop support XB1 at some point.
To me it makes sense to rip off the band aid now. By then most Xbox gamers will have a Series S or X, and if they don't, xcloud will also work on XB1 consoles by then and they can play Infinite that way (if their connection allows it). Just take the PR hit... I mean, when is their PR not being hit, just business as usual. I think you'd have an equal amount of people celebrating the final death of XB1.