The Southern Tier is not part of Appalachia..
sure, it might have economic similarities to Appalachia, but so does Detroit, Cleveland, probably parts of California, etc.
just being "similar" does not mean "the same place"..
Absolutely no one in the Southern Tier thinks they live in Appalachia, because they don't!
Im a Southern Tier native, from Waverly, and I come from 200 years of Southern Tier natives..
My Dad was with the very last group of people at the A&P plant in Horseheads..he started working there
in the mid-70's, and helped clear the place out when it shut down a decade later..he was then unemployed for two years when I was a teenager.
I think what caused the economic decline of the Southern Tier was simply changing transportation..
In the 1840's New York State had two primary east-west transportation corridors, the Erie Canal across the top of the state,
and the Erie Railroad across the Southern Tier..
The Erie Railroad was, for a time, a larger and stronger railroad than the various embryonic railroads that would eventually form the New York Central..
But in the 1840's, the Erie Railroad was essentially the equal of the Erie canal, and the two "corridors" were on equal footing, from a transportation perspective..
This created an early boom in the Southern Tier cities, Binghamton, Elmira, Corning, even Waverly, where LV coal interchanged with the Erie to get to Buffalo.
But this "equality" didn't last long..
Eventually the New York Central railroad far surpassed the Erie in size and influence, and the early Erie Canal, combined with the new New York Central,
caused the "upper" cities, Albany, Syracuse, Rochester, Buffalo, to far outgrow and outpace the Southern Tier cities..
The large size of Syracuse, Rochester and Buffalo, compared to the smaller size of Binghamton, Elmira and Corning is a direct result of these differences in transportation..
By 1900 it was already over, and the "fading" of the Southern Tier was set to begin.
The southern tier railroads lost power and influence, reducing their capacity to fuel an industrial base, and the major industries faded along with the railroads.
The most important rail corridor in the state remained the New York Central mainline, which is still FAR busier and important than the old Erie main today.
So..IMO the southern tier faded because its industrial corridor neighbor to the north was simply stronger.
The Erie Canal and the New York Central both benefited from better geography..the "Water Level Route" and the Mohawk River passage between the mountains..
and it was also directly connected to the states North-South transportation corridor, the Hudson River..while the Southern Tier corridor was not.
It was simply a better transportation route than the Southern Tier..better transportation equals better growth and better economic health.
The Southern Tier gave it a shot, and early on things looked good with the new Erie railroad, and early cities grew because of it..
but in the end it was "second best" and couldn't compete.