I don't think the second screen did them any favors, either. Very few games use it well enough to justify adding the expensive controller to the console's price. I think it was a mistake in that it meant they could never drop the price to a level where it would be competitive because they'd lose money.
For SURE, it was not the biggest mistake, tons of other factors added together made the wii u a failure, but the gamepad definitely didn't help it out any.
Can NX succeed if it's a Wii U 2? Possibly, but it's definitely going to jack the price of the system up if you need both a set top box AND an expensive controller with brand new free-form LED screens.