Speed is not the only metric with which we judge trains.
Does the NYC-DC route need to be faster? Not much; maybe a half hour is all that is needed. It has captured quite a share of the travel in between those two cities.
Instead, perhaps ride quality, frequency, and price matter much more than increasing speed.
That said... the NYC to DC Acela route is comparable to Milan-Venice Frecciarossa (The Frecciarossa widely considered the best HSR in Europe) route in speed and distance. You throw around all of this hand-waving exclaiming "unacceptable," but honestly, the NEC provides mostly decent rail service. I think your word unacceptable is actually misplaced. Could it be better? Of course. Given the way this country works, and given how little we invest in rail, I'm glad it isn't worse. It's not the Tokaido Shinkansen, but it isn't exactly bad either.
If the 1970's Boston plan to tear down the NEC and replace it with an interstate had gone through, we'd have no real NEC.