How so, if you don’t mind spoiling it?
I could see having an interesting conversation about what would have happened if JFK had been shot 3 years earlier, or if RFK hadn't been killed in 1968, or if Gerald Ford had been reelected. You might get a good short paper or sketch out of it, but I can't see carrying it much further than that.
Maybe it's just me. Maybe there's a big market for 448 page collections of 100+ page counterfactual histories. But I'm thinking in speculative stuff like this you can make the point in 20 pages or so, and the rest is just padding. Also, if I wanted to read for several hours about Lyndon Johnson, or Richard Nixon, or Gerald Ford, I'd probably want to read about the real one, not about a half-fictional one (though maybe a wholly fictionalized Ford would be more interesting than the real one).
If you know anything about Jeff Greenfield, you probably know what he'd think about a Robert Kennedy presidency. If you've participated in discussions about recent political history, you can probably guess what a Ford victory in 1976 would have meant. So the only one that's a real mystery is what would have happened if LBJ had taken office in 1961 instead of in 1963.
Finally, another good reason for keeping these things short is if they aren't, a lot of snarkiness and wish-fullfillment creeps in. The further the narrative gets away from reality, the more opportunities there are to reward one's friends and punish one's enemies, to settle scores and live out one's own dreams. For me, personally, I'd rather have read a bunch of shorter "what-if" pieces than try to slog through what Greenfield actually wrote. But it could be that's just me. Plus, if you call your novellas "stunning" in the title, you're asking for put-downs.