I think people are mistaking 3DS not having DS' astronomical sales, for 3DS selling badly. 3DS has on average, been selling over 100k per week. Some weeks, like the most recently tracked, it was well over 300k. A system selling over 50 million units worldwide, is not anything even remotely close to approaching "bad".
The fact of the matter is, DS and Wii both (and even PS3 and 360 for that matter) had massive boosts in console sales numbers because of the "casual" gamer phenomenon. Many of those people have simply moved on to what is conceivably cheaper and easier to access "casual" fare: that being shitty little games they can get on their phones and tablets, that have very little actual commitment to them, either in time or money.
I do not think that if Nintendo released another straight up portable, meaning that even if "NX" did NOT wind up being a "Fusion" device concept, I think it would sell well, and over time at least sell 3DS-like numbers, which is GOOD. DS type sales are probably not going to happen again. And if anyone wants to try and point back to Game Boy sales....well there are a couple of elements to that. For one, they released many different versions of the Game Boy over a longer period of time, and for another, GB had it's own "casual" type phenomenon in Tetris. Tetris in the late 80s and early 90s was a SUPER big deal, even to people who weren't traditionally "gamers", and many adults bought a Game Boy just because it came packed with Tetris. That was a genius move on Nintendo's part. And later, the Game Boy Pocket model got a massive boost from the Pokemon phenomenon that swept the planet. So two major gaming fads really helped boost those sales. As you saw, Game Boy Color and Advance models, while STILL selling quite well, the numbers started coming back down to Earth. It wasn't until the "casual" craze popped up with DS that number skyrocketed once more.
And the point ultimately is, that people were fools to ever expect that 3DS and Wii U would sell anything close to DS and Wii numbers. Granted, the Wii U has under-performed by any measure. But the 3DS has been a success for Nintendo, as all their portables (minus the Virtual Boy experiment) have been. And if the "NX" is a two-in-one type deal, doubling as a home console and portable, well.....that is going to skew how people view it's numbers. But as I said, even if it were JUST a portable, with no home element at all, it would likely continue to sell 3DS like numbers.