Extremely dumb question, but I'm curious, so...
What exactly is the purpose of NY 17?
It helps to go back to the time of the railroads, when transportation corridors were basically private ventures and control of them was the source of much competition. In the early days of the railroad, there was something of a race to connect New York City to the Great Lakes–
above the falls of Niagara, mind you. Vanderbilt's New York Central had control of the "water-level route", following the Hudson and Mohawk Rivers through their natural valleys, and then heading west through the relatively flat corridor enjoyed by the Erie Canal.
At the same time, the Erie Railroad had the idea that they could compete for this traffic by heading along the Susquehanna, Chemung and upper Allegheny Rivers, along with Lake Chautauqua, to meet Lake Erie well upstream of Buffalo. Why this route, through much more difficult terrain with sharper curves and steeper grades, was thought feasible is somewhat of a mystery in hindsight, but it was an optimistic time…
Anyway, long story short, the Erie route was eventually finished, but not before the water-level route had established its primacy as
the way to get between the city and the hinterlands. It's still the principal rail corridor, and was later chosen by the Thruway (and earlier by the canal) as well. Meanwhile, the Erie route has always been an available back door–and has always lagged along behind. The early canals along the Southern Tier route have all long vanished, the railroad was a late finisher, and the expressway conversion was equally slow to complete, being unfinished even to this day.
So maybe that confirms your question–perhaps that route never has made much sense as an end-to-end alternative, but it's also never been abandoned, despite the challenges of completion. Maybe it's nothing more than manifest destiny. :-)