What about a 20th century POD. Do you think there is a plausible scenario in which Boston enters a death spiral of demographic and economic decline like Buffalo, Detriot and Pittsburgh experienced during the last third of the 20th century? An earlier start to forced busing could have led to uglier and even more destructive rioting, which would cause even more middle and upper class families to flee to the suburbs.
The "Massachusetts Miracle" never occurs, and Boston in 2012 retains a prosperous core in the downtown, while the rest of the city falls into disrepair, with the population halving in the city proper even while the suburbs surrounding the city such as Newton remain prosperous. Landmarks such as Fenway Park and the New England medical center are isolated by blighted area known locally as The Combat Zone. Local leaders discuss urban renewal and legalized gambling, while Boston University and Northeastern University struggle to attract students, only to see them move south to new york, or West upon graduation.
The tech boom, when it occurs in the 1980s is further West, along 495 in central massachusetts instead of route 128. Many of the young engineers and professionals flock to the nearby City of Worcester, whose own fading downtown is revived. Driven by software, medical and high end manufacturing, Worcester county flourishes, with the city itself increasing its population by 75% between 1975 and 2005, leaving it with more than 300,000 souls, surpassing Boston as the largest city in Massachusetts and New England.
How plausible does that sound. Admittedly I would be more interested in seeing a scenario in which Providence predominates. Just imagine how the man who said "I am Providence" would react.