With big releases comes big swells in console sales. The only thing that held the Wii U back this gen from doing respectable numbers is the fact there were so many game droughts. When Zelda Wii U releases, we can expect another home console sales swell.
As for whether the Wii U is a money sink hole, I think the fact that their fiscal year financial report was in the green means that they can still support the Wii U just fine until 2017. I think it would be a good idea to do this as well being that every Wii U and game sold, Nintendo makes a profit. The worst thing they can do is rush a new console out instead of making sure whatever this new consoles "thing" is, it can be fully realized. They should have had a game blowout at E3 this year to spark some interest again, showing their hand for 2016 but that is in the past.
What they do need to do is make sure their home console is stupidly easy to port the definitive versions of third party games to the console. They need to make sure that Monolith Soft and EAD whatever (the one that's making the Mario NX game) have games ready to launch when the console releases. Being that Retro said making a Metroid game now would be ready when NX launches is an insight to what is actually happening and why we haven't heard what their game is (though I would be fine with learning it's Metroid Wii U) that they have been working on since Tropical Freeze.
And hell, a great starting place for the new Nintendo console would be to launch at $350 to keep the price similar to the PS4 while offering a "better experience".
These things would help out FAR more than just announcing a cross gen title of LoZ. I think some people are placing far to much importance on the series like it can carry a new console.
I don't place much importance into it, actually. I don't think Zelda can carry a console. I don't even think the series is as significant of a release as it was in the early to late 90s. Zelda ist just one of many games on the market. I merely see it as one of many hints that Nintendo's next console will be out in 2016 and I totally support this decision. No matter what you say, Nintendo can't survive without a home console business and in early 2017 there won't be any home console business left without a new system. That's what their internal research says, as well. Otherwise they'd launch the next handheld first, as they always did. What I think is that people place too much importance on "Nintendo fans". Whenever Nintendo tries to please its hardcore fans, it fails spectacularly in the market. Whenever Nintendo doesn't try to please an existing fanbase its sales skyrocket. That's the picture I see and you may disagree with it.
I doubt they can support the Wii U in 2017: 3DS sales numbers will go down as well and they cut costs as much as they could already. I'm sure that's what their internal projections show, as well.
The Wii U is a flawed system: the Gamepad never worked and never enticed consumers, the flash storage is extremely slow, third parties never liked the architecture and Nintendos first party studios struggled from the beginning with game development on the system. "Kill your darlings" is probably the most important piece of advice I've heard in my life: If something doesn't work and you've really tried to make it work, kill it. That's what Nintendo is doing with the Wii U (they really tried, but they couldn't integrate the gamepad in a way that improved the games) and I support their decision. But I can totally see where you're coming from.
And I agree about the third parties part and Mario / Metroid. Again, I can totally see your point. I think they are stuck between a rock and a hard place right now.