Music's a hard thing to write about like that, isn't it? Like, if he survived, a large part of what people are going to want to know about is how his career goes, and its hard to really fictionalize that and get it across, as most AH writers are not themselves accomplished musicians capable of putting together a 'Surviving John Lennon demo tape' for the reader to really gel the whole presentation.
It's kind of a part of the fact that a lot of AH is easier by subtraction rather than addition. I can tell you what might happen if some person who lived instead died, because I can subtract them from what's known, but its harder to do the opposite, since you're on much less firm ground there. Like, I think a lot about what it would be like if various lost works of history weren't lost, like, if, say, we found all 142 books of Livy's history of Rome, but how I can I really write about that when we don't know what was in them. I'd have to fabricate it completely, and that runs easily into all of my own biases and ignorance and preconceptions that will make it probably ludicrous for anyone who isn't me to read.