I do wonder if too little credit is being given to NSMB and Mario Kart for the success of the DS and Wii. It looks like they were just along for the ride after NSMBU failed to push the WiiU much, but NSMB and MKDS still look pretty vital to the DS' success looking back on it (and how the DS rose after NSMB).
Maybe NSMB lost a little appeal instead of not having any power and being along for the ride.
The DS and Wii NSMB games had a ton of power. Sure. NSMB Wii was sold out in some stores when it came out in the 2009 holiday season and Wii broke the december record. You bet it's got power.
The problem now is that it's not evolving. It's been a long time before a 2D Mario came out so when the DS and Wii games came, people welcomed them and excused the medicore music and graphics.
Now it's overkill. They released two "new" ones this year, separated by mere months, and they're barely different from each other or the ones on DS and Wii. Backgrounds are updated a bit but the artstyle still sucks and the character models in U look just like the Wii game's and the music is largely the same and even more annoying now. New levels alone don't cut it, though the levels mostly expand on old ideas. It's not like Super Mario Bros. 3 that got rid of flag poles and threw in air ships and ground battle tanks and laser eyed statues among a ton of other things and expanded the Mushroom Kingdom with the eight worlds we play in today complete with kings and wands, and Peach wasn't even kidnapped until the end.
I think they're hurting 2D Mario by being idiots with how the series is being managed. I think people do want more 2D Mario, but they don't want more "NEW" Super Mario Bros and "BAH BAH" and all of that.
That's why I think the Wii U and 3DS games will be a large decline from the Wii and DS games, where as Call of Duty's decline is small and... heck, Black Ops and Modern Warfare are seen as two different brands of Call of Duty with Black Ops now being the more popular of the two. I guess it also helps that there all on PS3 and Xbox 360, selling to an existing userbase and expanding on it. NSMB on 3DS and Wii U have to sell systems, but NSMB Wii did have a large Wii userbase to sell to when it came out. NSMB DS' sales came over time.