Roger speculates on how things might have been different had Mark David Chapman been somewhere else that night in 1980:
John and Yoko’s album Double Fantasy comes out in the fall of 1980. It does all right [not as well as it did in response to Lennon's death]. They put out Milk and Honey a year later; ditto. They tour for a few months.
Around 1982, George, whose career was in a bit of a downturn — no “All Those Years Ago” hit single — plays on a John and Yoko album. John and George play on Ringo’s comeback album.
Live Aid in 1985 becomes the venue in which the Beatles get together for a one-off reunion. But they enjoy it so much, they put together an album a year later. They get together periodically, but primarily continue with their solo careers.
Roger doesn’t say so specifically, but it sounds to me like the stumbling block, had John lived, might have been Paul. (How does he sleep?)
As long as we’re fiddling about with timelines, you might have a look at “The plane that didn’t crash,” based on an earlier catastrophe.