Depends on if the PS4 and\or XBox3 will have a comparable (or better) tablet. If not, the WiiU will always have the tablet over the others. The PS4 promises interactivity between the Vita and smart phones. Smart phones are increasingly becoming a staple, so being able to interact with a console would be a huge leap in usability. Will smart phones interact with the PS4 and TV screen in the same way the WiiU tablet does? Hope so, far from guaranteed. Sounds more like Sony's doing an OnLive thing with them. Perhaps the XBox3 will be able to use smart phones (including iOS and Android, not just Windows devices) in ways like the WiiU's tablet; that would all but remove Nintendo's edge here. Probably not all that likely, but if any console could integrate with iOS and Android phones to the extent of WiiU tablets, 4 or more at a time, I'd bet on that console leading the generation by a good margin as long as it was ok otherwise.
Also depends on how cheap Nintendo can afford to make the WiiU in comparison of the others. I very much encourage powerful consoles that can bring the power of not-too-old gaming PCs, yet it still should give Nintendo an edge on console price, even counting the extra cost of making the tablet. If they can leverage that, they should sell enough to keep a place in the market, even if the weak specs are a deal-breaker for many.
Finally, while it's up to personal preference, Nintendo is still making games that some people find great. Even New Super Mario Bros. U, which was clearly a quick game they knew they could have ready for launch, and the fourth in the NSMB series, is a lot of fun to play, both for the level design, and the fifth player that gets to place blocks with the tablet. With successes like Super Mario Galaxy 1 & 2 in their recent history, the hope is that the WiiU's library will have some further winners over the years. So with any luck it will make reasonable sales and achieve that.
The 8th generation of console gaming should be a good one. With the "big three", the Ouya and other Android boxes, Steam Boxes, increasingly easy PC-TV integration, and better tools for porting games across platforms, there should be something for everyone, and a lot of surprises along the way.