Well the biggest problem is that Nintendo's solution is to throw more Mario at the problem. It's sort of worked for them the past 30 years. But it's not gonna work anymore. I agree Nintendo needs new blood. Or rather, they need a better NA division. Making Retro work on another Donkey Kong is probably the worst decision they've made these past few years. Nintendo needs new creation, not sequels.
it actually never worked for them in recent gens. it didnt work for the N64, it didnt work for the gamecube. the gameboy sold because of pokemon, as well as the gameboy advanced. the DS sold because of pokemon and nintendo dogs, the dual screens and such. the Wii sold because of motion controls and Wii sports. Mario just rode the hype wave for those systems.
i firmly believe that now Mario sells based on the current platform total sales. Wii sold gagbusters? so mario sold a lot. mario is the kind of game you pick up if you already have the console. not the kind of game people pick up a console for (talking about the average consumer, not nintendo's dedicated fanbase). if mario released on playstation, wouldnt you pick it up? Mario bros on Wii sold 27+ million. it will sell what on the WiiU, 6 or 7 million with some massive bundling? same with the DS version, sold almost 30M. it will sell like 10 million on 3DS. a massive drop in user base results in a massive drop for game sales.
in comparison look at GTA sales, it has similar sales, but is consistant. GTA vice city sold what, 22 million? san andreas 27 million? GTA4 25 million and now GTA5 32 million. they always post mega sales because its a tittle that people feel that they need to play, mario simply isnt anymore.