If the Xbox One really takes off and runs with the PS4, and leaves the wiiu significantly behind, then I would say the WiiUs goose will be cooked around late 2017 to early2018

I think Nintendo will ride this out for six years so 2018 I think we will see an announcement and release a la the PS4/Xbone. Now that the Wii U seems to have reached a comfortable level of profitability (we'll have to wait and see to be sure but it seems to be getting there), I think Nintendo will want to recoup losses from early in the gen. Also, while having no 3rd party support burdens the company it also let's them flex their muscles as a developer; I think they will follow up what they did at this years E3 with more gamer-focused press events to build the company's reputation and improve awareness of the depth of their IP. I think we will see more new IP as well as more promotion of their core-gamer-orriented IP (like Xenoblade, Metroid, etc) to achieve this end. I also expect to see a number of IP get representation on Wii U (I am banking on F-Zero's return personally).   Also, 2018 would give Nintendo a lot of room to develop a stellar launch title like a Zelda or full 3D Mario supported by one of their younger IP that is more core orriented like Xenoblade. 

Even if the max they can achieve sales wise is between GC and Xbox original sales, if they can come out with a much stronger image, it will be to the benefit of their new console. Plus, waiting until 2018 guarantees they can easily blow past the PS4's power by a substantial margin and keep the starting price reasonable.

Wii u!

I think the last year of meaningful software support will be 2016, and that Wii U support will dry up in the first four to six months of 2017, for a replacement Christmas 2017. Nintendo need to get right whatever they launch next, and cutting and burning Wii U too soon will alienate part of their already small core fanbase.

Anybody suggesting Nintendo cut and run and move quickly need to look at mid-90s Sega.

if the nintendo actually tried to advertise the wii u ... it will sell more

If it releases holiday 2016, it wouldn't necessarily need to be announced at E3 2015. Looking at PS4/XB1, it's entirely possible to announce a console and release it in the same year. Nintendo could announce it in a Nintendo Direct in Feb, give a full blown reveal at E3 in June then launch it in Nov.

The Sega analogy isn't really the best, they kept releasing unsupported devices throughout the 90's. Sega Game Gear-1991, Sega CD-1992, Sega 32x-1994, Sega Saturn-1995, Sega Nomad-1995 all discontinued within a few years and had minimal support.

A better example would be the original Xbox or Gameboy Advance which each were replaced within 4 years and the successors went on to sell much better. If Nintendo supports Wii U to the end and releases a well thought out console at an affordable price with strong software support then I doubt their will be much backlash with a holiday 2016 release.

By this time Wii U will have 2D Mario, Nintendo Land, Lego City, Game & Wario, Pikmin, Wonderful 101, Wii Party, Wind Waker HD, 3D Mario, Donkey Kong, Wii Fit, Wii Sports, NES Remix, Captain Toad, Mario Maker, Mario Kart, Hyrule Warriors, Bayonetta 1+2, Devil's Third, SMTxFire Emblem, Project Gaurd/Giant Robot, Kirby, Yoshi, Xenoblade, Star Fox, Splatoon, Mario Party, New Zelda, Fatal Frame, Monster Hunter, Mario & Sonic Olympics, Sonic Lost World, Sonic Boom plus likely a few major unannounced games like Animal Crossing, Metroid, Paper Mario, Pokken Fighters. U can't say Nintendo didn't give strong support to Wii U over the course of 4 years with a lineup like that.

Besides what exactly does Nintendo gain from dragging out Wii U an extra year? An extra 2 million sales and a long 2017 drought?

nintendo has done well with what they have but I believe with the latest games and one or two on the way it looks like they have maximized the potential of what is wiiu now. It will be smart of them to have a dev team in the back ground getting together some launch games for the next gen of nintendo hardware. The HD transition shouldnt be such an excuse/problem now.

Ya especially since Iwata said they want the next Nintendo devices to "absorb" the Wii U architecture, in order to have a seamless transition next-gen. If they still have a problem with getting games out in a timely manner then there is something wrong.

All of that next year, when they are just now hiring guys with experience to help them choose what parts they need to put in the new Nintendo console. I just don't see it happening. 

http://metro.co.uk/2014/10/22/nintendo-hiring-for-next-gen-console-4916437/

Take into account Amiibo hasn't even came out, nor has the Quality Of Life stuff. 2017 earliest, mark my words. I'm actually thinking it will be more 2018 time frame, but I think Nintendo could benefit from releasing it in 2017 due to being able to release a console with similar specs (hopefully slightly above) of the PS4 and still have a good few years before the competition's machines (thinking PS5 new XBox will release 2020).