Do you mean NINTENDO'S core market, or the rest of the core market? Not to say there isn't overlap in people that enjoy both, but the rest of the core market will obviously be getting either the next-gen platforms or be running with a PC. Attempting some kind of parity between them and the PS4/Durango's release calendar can't be a realistic move for them. It sure as hell didn't work out on the Gamecube, and that was a technically comparable console.
I can see your argument if you mean that Nintendo needs to push their IPs hard, though. Like "here are your Nintendo-ass Nintendo games. A great charming awesome Zelda, a brilliant 3D Mario, a badass creepy Metroid."
Eh, I wouldn't be entirely surprised to see business pick up among the 'casual core' if brand awareness increases exponentially and Sony / MS decide the cut their existing consoles' legs off at the knees while a price-cut Wii U continues to get lower-end versions of the usual suspects.
It's really all about the brew of brand + price + software. There were an awful lot of multi- format of games that offered little reason to be played in their janky PS2 forms, but people weren't interested in migrating to Xbox (or, to a lesser extent, GameCube) and I don't think they would have done if GTA had release parity.
Note that I'm not saying there's going to be a mass-exodus of CoD and sports gamers to Nintendo, but it's a fluid market and a quirky generation, so I'm not convinced there's going to be astonishing demand for the new machines at launch. It would, of course, be very foolish for Sony / MS to attempt to nudge people away from consoles that really didn't have great traction and instead flourished over an extended period of time.