The Atlantic asks Harry Turtledove, writer of counterfactual histories. He points out that, in fact, the alternatives are written not to be realistic, but to throw interesting dramatic light on the world we do inhabit.
Alternate history isn't really about the world you're creating," he added. "It's about the world in which you live, and gives you and your readers a funhouse mirror in which to see the real world." It's a reflection, he says, that we can't get any other way.
In other words, it's SF. When pressed, his suggestion isn't particularly edifying:
"If the British Empire included all of North America north of the Rio Grande as well as India, it would be incontestably the strongest state in the world," he responded. "The French Revolution wouldn't have happened, both for lack of example and because it began when a political crisis and a famine coincided with a government bankruptcy that sprang from the money the government paid out helping the American colonists gain their independence—and giving perfidious Albion a shot in the eye."
Or perhaps it would merely have been delayed, because the causes of the French Revolution ran deep.
Sans American revolutionary success, my money is that the world would be much as it is now. America would just have become independent anyway, as Canada did, in a slower and less constitutionally dramatic way. How awesome would THE DOMINION OF AMERICA have been? Quite.
Here's one for the mental hopper: a comedy where, the morning after dropping a twinkie wrapper in a new particle accelerator, U.S. President Bill Murray wakes up to find that he is now U.S. Prime Minister Bill Murray, and must navigate his ignorance of the tiny-yet-critical details that have changed--and learn how to restore the reality he's used to.
This would not only be a comedy of errors with broad Transatlatic appeal, but would provide employment for science writers who like to point out the paradoxical absurdities within movies featuring time travel.