Nintendo makes family-friendly games which anyone can play and enjoy regardless of age. This is by definition more mainstream than the realistic war/action games on PS3/360.
Nintendo's dabbled with those types of games on GC (Eternal Darkness, Geist) and Wii (Xenoblade, The Last Story, couple others only released in Japan) but the mainstream audience don't buy Nintendo consoles for those types of games and people who like those types of games will buy 360/PS3 (or their next gen counterparts) regardless of WiiU having a handful of exclusives (like Bayonetta 2). Because at it's core Nintendo will always be more family friendly / mainstream than the others who cater primarily to core gamers.
WiiU's problem is it's not resonating with the mainstream, casual, family audience. Nor the core. It frustrates me, because Nintendo made the wiimote to regain this audience because the modern controller had gotten too complex. It worked!! So why they then go back to the very thing (dual analog, tons of buttons) that seperated them from their market is beyond me. If the gamepad was a true tablet, I'd get it. I'd buy it and so would a ton of others. But a limited touchscreen slapped on a standard controller? No one wanted that.
Re: Zelda I argue this with my (adult) son all the time. He wants a gritty, realistic Zelda too, but I remind him I had to help him play Z:OoT because the zombies and such scared him too much. So why make a Zelda game that kids can't/won't play? That's not what Nintendo has every been about, nor Zelda. Zelda should always be 'cartoony' enough so that a new generation can discover it and enjoy it just like all of you once did when you were young.