Nintendo's console decline in the 90s/early 2000s was masked by the Pokemon revival of the Game Boy and the sudden ramp up of the portable market which peaked with the DS it looks like.
Smartphones/tablets really screwed Nintendo. They'd have nothing to worry about if those didn't exist, but I guess Blackberry would say the same thing, lol.
That said I think Nintendo will make a fortune off smartphone apps over the next gen so much so that I think NX merely has to do "ok".
If they can sell 60 million NX units, shared between the console and portable, even that would be "ok".