Thing is, Twilight Princess is the only game in the series to sell the numbers that it did. Everything else is usually somewhere between 3-5 million. The only games to exceed that are Phantom Hourglass (barely), Zelda 1, Ocarina of Time, and Twilight Princess. So, given the patterns of I don't think they were expecting those huge numbers from Skyward Sword, which was released when the Wii was all but dead and they were moving on to the Wii U.

I hope not. I want them to keep pushing farther this new formula.

How is it denial when we have records of sales data to back it up. It happened with Zelda 2 from Zelda 1 (3 million less) Majora's Mask from Ocarina of Time (4 million less). Spirit Tracks from Phantom Hourglass (1.7 million less). 
It even happens with their other franchises. 

Galaxy 2 sold around 4-5 million less than Galaxy 1 (HUGE drop). Handhelds that have 2 mainline Pokemon games usually follow the pattern of the 2nd ones selling less than the 1st ones (Gold/Silver sold 10 million less than Red/Blue; And Black/White sold 2 million less than Diamond/Pearl). Though, Sun/Moon looks to be the exception seeing as how that had Pokemon GO hype behind it + the series' 20th Anniversary.

If they were expecting gangbuster, 7-9 million numbers from Skyward Sword, now THAT'S delusional. And THAT'S being in denial. Because the game had many aspects going against it when it launched. So the fact that it STILL managed 4 million is a testament to the legacy, reception, and huge amount of respect that the Zelda franchise has accumulated throughout its lifespan.

With that said, I agree that if Breath of the Wild only managed to accumulate 4-5 million on two systems, that would be hugely dissappointing. Because it has a lot of momentum on its side. 

They will probably keep this formula but will probably make a very different game next time. Very different location, dungeon scheme, themes, etc. Maybe this time Link will be able to travel to the future? I mean, not our future, but Hyrule's/Termina's/Wherever's future. Maybe a world dominated by sheikah/hylian tech, kinda like a distopia, mixing elements from future and past. I mean, they've been experimenting with advanced technology on Zelda for quite a while. The first 3D game I remember having these elements was Twilight Princess, which features cannons, motion controlled statues and a freaking city in the sky! Then Skyward came with Robots and stuff, and now we even have automated creatures (even automated "divinities") shooting lasers and stuff! So... it would actually be cool to have a game partially set in the "future" next time.

It may not be satisfactory, but what'd' they think was gonna happen? Like I said, it had a bunch of circumstances going against it when it launched that expecting huge, Twilight Princess like numbers from it was unrealistic and delusional. It just, was, not, going, to, happen. The only reason one would have for expecting it to happen was blind optimism. So, the fact that it managed 4 million. Which is STILL above series standards and profitable should be seen as a "Take what you can get," circumstance. Because 4 million is nothing to sneeze at when it comes to Zelda.

I brought up those games to illustrate the pattern that Skyward Sword was destined to follow: Development costs or not. Even if it wasn't what they were hoping, it was still good and profitable.

In a recent interview they stated that a 2D zelda is definitely possible on the Switch.

http://www.gameinformer.com/b/features/archive/2017/02/14/a-2d-zelda-on-the-switch-is-definitely-a-possibility.aspx

That would be awesome but I have my doubts as to it actually happening.

They'd have to really spend a lot of money into expanding their game development staff to pump at that many more games. (Though they're more than capable of spending the money and in the long run, it would be a great thing for them and for us.)

Again as someone previously mentioned, Nintendo always wanted Zelda to be a big open world experiance they just never had the hardware that could pull it off, so in games like Windwaker they gave you the illusion of playing in a big world, all be it, it was mainly an ocean with islands dotted about the map.
This will be the formula going forward, we can expect different art styles and maybe out of the ordinary worlds, who knows.

Came here to say this. I fully expect them to put trains, birds, or linear gameplay (SS) into the next Zelda. I really want them to make BotW 2 though. There's so many worlds you can build based on this game, so their usual apology of "we had to do something new" won't count. They can even re-use ideas from older Zelda games and they'll shine in a new light in an open world game:

- An open sea adventure (make diving possible and the bottom of the ocean can be explored as well)

- Light / Dark World

- Time Cycle

- Transformations 

There's no need to do strange experiments at this point. But who am I kidding? This Zelda was well received so they'll feel free to go nuts again.