I can totally see Zelda slipping launch. It just makes a lot of sense for Nintendo.
1. Zelda will sell - no matter when
2. They give Skyrim a little breathing room, and Skyrim on a portable is a bigger deal than a lot of us are thinking. It's the most beloved open World game next to GTAV and it's sales would be squandered if it launched next to Zelda. And Bethesda is a big 3rd party to court.
3. No way they are launching Zelda and Mario at the same time!
4. They have an easy option with to expand their launch window lineup that's way more mainstream than Pikmin (as good as that is)
5. They make good fast on a steady release stream
6. May has proven to be a great release spot for a Nintendo evergreen
IF Mario is there on launch day together with Skyrim, Rabbids RPG, Splatoon and Just Dance, they check a lot of boxes. From Open World, to Nintendo Powerhouse, to Online-Multiplayer to quirky Idea hopefully executed well.
IF they then follow up with Mario Kart a month later and Zelda a month after that... It indicates a steady release stream and these are games that will sell and attract different kind of crowds. And that's not talking about Pokemon, Smash, Pikmin and maybe Mario Maker that are all on the horizon for 2017.
IF those all make it, that's pretty much already an All-Star Lineup with a good stream of releases that will surely be build upon by games we know nothing about yet, Indies and 3rd Parties. It's easier than ever for Nintendo to make a great first impression. It's their part to fumble now.