I'll get straight to the point:
If that is the case, then explain why the current sales are low considering it has 2D Mario. The key is "games that interest them". Are enough people still interested in the Nintendo franchises? 

GC sold how many exactly? 21 million units? As you seem to be digressing down a different path, let me reign you back into the OP who is claiming the Wii U isn't doomed to third place. If the adoption rate is  ~30% for mario kart, you are then, at most, going to see, when it's released *whenever* only a ~33% adoption rate. At the Wii U's current pace, you are going to need to need a phenomanal adoption rate (think 500%) for the console to hit the 20 mill mark. 

Also don't leave out NSMB Wii. That one's interesting as well. 

2D mario launched with the system didn't it? That's one game that launched months ago, and has sold 1.6 million units according to VGC... Thats pretty impressive given the install base, and besides, thats a single launch game. SSB interests the core. Mario Kart interests the Casual. When more titles come out, sales will rise. Games sells Hardware, he is corect, so what exactly is your point?

I for one whave never bought into the new 2d mario, and I'm waiting unti I can warrant spending $300 on a WiiU... Aka when the bigger games come out for it, and more than just the one game you mentioned.

It comes bundled in a lot of places...
Does SSB still interest the core? Will it really offer enough so that people buy an entirely new console just for one game, when, lets be honest, they'll most likely only be able to choose between the PS4/720/Wii U this year? If you take your average core gamer, show him Shadow Fall and SSBB, I'm going to guess he'll choose the former, not the latter...
Are casuals really interested in upgrading? Casuals don't upgrade; they just move on the next cool thing. 

The PS3 managed to sell with absolutely crap games - lair, resistance and motorstorm...

you fail to understand that the marketing by Nintendo was really really poor, every Nintendo-fan can tell you this. The ps3 was hyped. very very hyped. and it still didn't took of till the price drop.

the new super mario bros. series isn't a system steller it looks like. it's more a game the people buy when they see it randomly in a store. cause super mario is one fo the few games everybody can play. the huge sales are proofing this. mario u has a very impressive attach rate if you are honest with you. and you think 11 million brwal owners just lost interest? just so? you could have said that if PSABR would have been successful. smash bros is a franchise nobody copied successful.

and what a dumb dumb comparison lol, comparing brawl to shadowfall. comüarin previous gen to next gen games haha. you talk out of your ass lol. we didn't even see footage from the new smash bros.

EDIT: btw, i never saw a super mario bros u. bundle. and ps 3 had still the HD, blu-ray bonus. i guarantee you, many will be just hyped to see nintendo franchises finally in HD. i get wet if i imagine a super mario 64 type overworld or openworld with the masterful level design from the galaxy games. all in hd.

The GameCube had Smash Bros. and Super Mario and Zelda and Mario Kart and Pikmin and Resident Evil too.

The reason the Wii sold so much more is because the Wii appealed to more than just Nintendo fans because of Wii Sports/Wii Fit and then the nostalgia of people wanting to play 2D Mario again like they did in the 80s.

But IMO both of those wells have dried up for Nintendo. Casuals have moved on to smartphones and tablets and aren't coming back now that they're used to getting arcade type gaming fixes for 99 cents a pop that fit into their life style better than a dedicated home game console they have to spend $300 for.

What hardcore gamers don't understand is casuals don't want to sit down and play games for hours and hours on end. The have sweet tooth for gaming once in a while, but that's something a short 10-20 minutes a day (if that) can be satisfied by cheapo smartphone or tablet devices, which are more convienant anyway, because you can play them on the bus/train, during the lunch break, in any room of the house, etc. 

The vast majority of Wii users did not purchase or care for Zelda or Smash Bros or 3D Mario. 

And the appeal of 2D Mario is normalizing because it was special when NSMB1/Wii came out as many people hadn't played a 2D Mario in like 10+ years, but it's not a big deal anymore when you already played the last 2D Mario just 2-3 years ago.

You presume that the PS3 sold just because of marketing. What about bluray?

Also the new super mario bros is often bundled, therefore the attach rate is irrelevant - it just shows it isn't a system seller. 
As I said, if it's the difference between the next Halo/Killzone/Uncharted etc game or SSB....yeah I think many people will lose interest. 

* I meant SSB. Don't be pedantic. 

you know, the gamecube "failed" 1. cause of it's design and the color. 2. the disc-format 3. the monster called ps 2 and a new competitor called xbox. the n64 wsold also only 30 million cause of cartridges. in a time where dvd and cd's were totally the shit this was a huge fail by Nintendo

btw i never saw a bundle with new super mario bros u. atleast in my country, and there isn't even an official bundle. mabye some stores but official there is no mario bundle.....

Matzy....lol Blu-Ray wasn't a system seller in the early days. few people had a hd tv and bul-rays were expensive back then. double the price from a dvd.

not so.  Playstation systems have always been bought for their exclusives from what I've seen.  Everyone I talk to says they buy playstation whatever for x game(s).  They would mention that it's nice to have a DVD/Bluray player but never was it the primary reason.