It's not like games with different settings require dramatically more resources. Look at PC games ... they usually have 4-5 different configs for vastly different hardware setups that run usually at 3-4 different screen resolutions:

https://www.youtube.com/watch?v=Hvp3UStDJUg

It doesn't mean the PC version of Crysis 3 suddenly required a ton more development time than the fixed format versions (console). 

This is also a quote from Mr. Miyamoto from last summer:

http://www.neogaf.com/forum/showthread.php?t=843149

Miyamoto: [...] but if you have a more unified development environment and you're able to make one game that runs on both systems instead of having to make a game for each system, that's an area of opportunity for us. 

The number of Nintendo published titles per year has stayed pretty consistant for multiple generations despite increased time/costs in game development. 1998, 2003, 2008, 2013 all were in the 20-30 range. Unless u think developing games for Nintendo 64+Gameboy Color is roughly the same as developing for Wii U+3DS than there is no reason to believe Nintendo will drop their 25 or so published titles per year average.

This is kinda circular logic though, because then according to your own scenario ... how then is Nintendo supposed to support two distinct platforms when we factor in higher dev costs/dev times next-gen?

By that metric, a unified platform is literally Nintendo's only option, they can't support two distinct platforms any longer going forward. They can barely manage as is with the Wii U and 3DS, if the 3DS was as powerful as the XBox 360 even (ball park), the situation would be completely unworkable just by bumping the portable spec up (even if you leave the console side alone as is). 

If they can only deliver 15-16 games/year as you presuppose, then a unified platform is really their only option. You can't support two distinct platforms at that output. 

They can't, which is why they get third parties to help.

What's the appeal to third parties?

I'm sure they will have similar support to the 3DS, which is OK, but primarily Nintendo is going to have to carry the platform on their own no matter what. 

Reducing software redundcy will be big for Nintendo too. We really don't need NSMB2 and NSMBU or Smash Bros. U and 3DS or Mario 3D Land and Mario 3D World, etc. in a unified scenario, Nintendo can make one version of their big tent pole games and then that "box" is checked, you don't have to worry about the "well we just launched 3DS, we really need to make sure it has a 2D Mario, and 3D Mario, etc. ... oh now we launched Wii U, now we need to do the same thing again for the Wii U". 

Just make Mario 3D World for both, then you can move that team onto something else, for example taking a chance on an original IP or taking a swing at another Nintendo IP. That broadens their software diversity. 

The software requirements for a handheld have changed dramatically for Nintendo over the years too. In the Game Boy days, the Game Boy got 2D Mario games and one 2D Zelda along with some late downports/remixes of the DKC games. 

But today the handheld line has become the primary hardware driver for Nintendo's business and as such it requires equal software offerings that the console does. It needs its own 3D Mario. And 2D Mario. And Mario Kart. And Smash Brothers. And Animal Crossing. Looking at those MM/OoT sales, its own 3D Zelda next time out probably is well warranted too. 

This is not like the 90s/early 2000s anymore where Nintendo could maybe throw out a handful of Game Boy games a game and call it a day for supporting their handheld while giving all the "big" games to their consoles. 

They have been able to keep their output rather consistant each generation, but now were getting to the point where were seeing that decrease, Zelda U is taking way longer than the other Zelda games, Retro came out with 2 games last gen, but the game they made this gen, DKTF took 5 years (Not saying they might not squeeze in another game but you can see that increase in development time).  This gen they've somewhat been able release as much big games as one would expect from a Nintendo generation, but its not gonna last forever.  

Because Nintendo's software doesn't have the wide appeal they need, or at least not enough to justify a console purchase.  That's where third parties come in.  

Exactly. 

So unified platform is their best/most reasonable option here. 

There's also the issue of bad business here ... your biggest games (ie: Zelda U which is probably going to be the most expensive game to develop in Nintendo's history) should be available where your biggest audience is -- and for Nintendo that's on handheld by a factor of 4x-5x. 

The setup Nintendo has right now quite honestly is ass backwards. 

Right now many of their best games are unplayable to about 80% of the people who buy Nintendo hardware ... because they're not willing to buy a Nintendo console, which really sucks for the people who work so hard on games like Bayonetta 2, Captain Toad, Pikmin 3, and even Mario Kart 8 .... these games deserve to be played by a lot more people. The vast majority of the Nintendo fanbase is locked out from these games. 

There isn't really much evidence that third party games significantly boost Nintendo hardware sales really. The GameCube and Wii U early on had a fair number of big third party IP ... Wii U had COD, Assassin's Creed, Batman, Zombi U, etc. and the launch was still extremely weak. 

Sony/MS simply have a strangle hold on the market that games like COD and GTA and Madden NFL appeal to and there's not a lot Nintendo can do about it unless they're willing to waste a ton of money/resources getting into a pissing match with the likes of Microsoft and Sony ... which they're not. 

So that's a dead end. 

If they did this, only the really hardcore people are gonna buy both the console and handheld, most people are just gonna buy the cheaper option, which is gonna be the handheld and their console sales are gonna plummet even lower than the Wii U.  In this cause they have to sell their big AAA Zelda U for $40 on handheld and make way less money per copy sold.  For every pro theres another con.