The 34 million people that bought Mario Kart Wii didn't all buy a Gamecube for its Mario Kart game. Does Mario Kart sell systems these days or is it more like people bought a Wii for Wii Sports and then, already owning a Wii, bought Mario Kart for it? NSMB is one of the biggest hits on both the DS and Wii but the Wii U launched with a game in the same series and the buying public did not give a damn. The Wii U isn't going to sell on the same gameplay experiences as the Wii. The Cube had all the big Nintendo franchises and it struggled. The Wii was different because it had a unique experience in Wii Sports that made it so that people felt they had to buy a Wii or they would be missing out. I'm sure the next Mario Kart will be a great game but no one NEEDS a Wii U to play Mario Kart.
You compared the Wii U to the Dreamcast though, not the Gamecube. The Gamecube still made Nintendo money in the end and had a full 5 year lifecycle. Of course even in that case, you don't seem to realize just how much more popular the Mario Kart franchise is now then it was back then. Mario Kart 64 sold over 9 million copies on the N64. In comparison Double Dash sold over 7 million on the Gamecube. Yes the Gamecube still struggled but Mario Kart was still a major seller and didn't see that big a drop between the two systems.
Last gen Mario Kart DS sold over 25 million and Mario Kart Wii over 34 million. As of the end of March 2013, Mario Kart 7 for the 3DS has currently sold over 8 million in a little more then one year. So in less then 1.5 years, the most recent Mario Kart for a handheld system, which is less popular then the home console games has surpassed Double Dash lifetime total and on it's way to surpassing Mario Kart 64's lifetime total. If the 3DS game can do that, the Wii U version is guaranteed to easily surpass the 10 million mark, which will be more then the Dreamcast could sell total.
So do you see the point? Comparing the Wii U to the Dreamcast is beyond foolish when Nintendo has series that can sell more games alone then Sega could sell consoles, meaning the Wii U is guaranteed sales much higher then the Dreamcast could ever hope to achieve. Plus Nintendo's big franchises are still more popular now then ever before which guarantees sales much higher then the Gamecube could achieve either.
And stop bringing up NSMB U since I've already mentioned that before. Nintendo releasing the 3DS game months before the Wii U took away a lot of attention since people could get a similar experience for a much cheaper console. If a NSMB fan wanted to buy the newest game and didn't own a 3DS or Wii U, the choices were spend over $200 for the 3DS game or $400 for the Wii U game. Or if somebody already owned a 3DS the choices were spend $40 for NSMB 2 or $400 for NSMB U. The fact the 3DS version has sold over 6 million compared to the Wii U's 2 million pretty much proves the choice people made.
In comparison, it'll be at least 2 years when Mario Kart U is released this holiday and the Mario Kart series is way more popular on home consoles then handhelds. Mario Kart Wii sold close to 10 million more then Mario Kart DS, versus NSMB DS selling 2 million more then NSMB Wii. So once again, Mario Kart alone guarantees the system is far from dead since it's one of the biggest series in gaming that is more popular then ever right now with the home console games being what most people prefer.