I saw a movie recently about the growth of Motown Records in Detroit during the '50s and '60s, and one of the points made was that Detroiters were deflated when Motown decided to move to LA in 1971. The thought occurred to me: if Motown had stayed, Detroit could've become to the national R&B music industry what Nashville is to country, and that could've been the post-industrial economic boost that would've completely altered the city's fortunes. Imagine: if you wanna make it big in R&B (or later, rap), you gotta go to Detroit -- singers, musicians, producers, choreographers, sound technicians, music executives, etc.
Surely there are other pivotal moments in other cities. What if they did or did not happen? Can you think of some for your city of choice?