Zelda is an evergreen launch title for the switch much like many of such titles on the 3DS it won't dry up simply because Mario Kart and Splatoon launch because they serve an entirely different genres and sets of users who likely won't buy the console just for one game. Even a year down the line if your picking up the switch you'll probably end up picking up Zelda too at one point. We'll see in a few years down the line but I think it'll be far more competitive than your expecting.
That's why I'm personally expecting Zelda BotW to have such a good performance in Japan. It's actually a title that compliments the multiplayer titles which are coming out during the Switch's first 6 months. You could buy Zelda with MK8D if that's when you are buying the Switch, buy it with Arms or buy it with Splatoon 2. I predicted around 1/3 of Switch owners to buy it in 2017. Personally I'm expecting it to get a bump when MK8D hits and another smaller boost when Arms releases and a really strong performance around the Splatoon 2 launch and summer holidays.
As you said it's an evergreen title for the Switch and the first such experience on a handheld device.