As much as i want to play Zelda, Switch needs Mario at launch more than it needs Zelda. And the reason is... Pokémon GO.
Look at what Pokémon GO has done not just for 3DS HW sales but how it helped push the new Pokémon games into most pre-ordered games in Nintendo history.
With Super Mario Run coming out in December, if a similar phenomenon happens - and with Switch, along with a Mario game, coming out in just 3 months after Super Mario Run - the console will fly off shelves... because of Mario, not Zelda.
Even if Zelda is not a launch title you can bet it will be out in Summer '17.