Ive stated this in the past a couple of times.
Nintendo consoles have been selling less than the previous iteration since the NES (except the Wii).
NES: 61.91 > SNES: 49.10 > N64: 32.93 > GC: 21.74 < Wii: 101.17 > Wii U: 10.14
The Wii was beast, but also a fad.
There are 3 scenarios possible with the NX imo:
1) The Almost Impossible scenario: The NX becomes a Wii 2 and captivates the harcores and the casuals.
2) The Least Probable Scenario: The NX sell less than the Wii U. I mean, history wise it would happen this, but the Wii U is just too much of a fail to be topped. It had to be a Virtual Boy like flop.
3) The Most Probable Scenario: The NX sells something between the Wii U and Gamecube.