sony and m$ arent pushing anything.  SNK was back in the 90tits.

Xbox's CPU was not state of the art.

And Gamecube had advantages of its own; faster RAM, more CPU cache, a GPU better at performing effects like multitexturing.

So 10 year old Apple computers had DX11 features?

PC in the loose term. Today, modern pc's are built with an accepted infrastructure that both Sony and Microsoft implemented in their consoles, literally making them almost medium range pc's. Wii U uses a different infrastructure that is harder to learn, however, just as capable. Though it only has three cores, the efficiency of the system as a whole makes anymore cores unneeded. 

However, Nintendo's deviation from the norm PC architecture allows them to have an incredibly efficient system. The power the Wii U can draw out of so little hardware and so little space just shows the ability of creating efficiency within hardware. 

Are there any particular inefficiencies in XBone/PS4 that Wii U overcomes?

Nintendo's hardware strategy is to remain profitable or at least close to it and if that means sacrifices must be made then the mindset seems to be to reduce power before anything else. Like the OP said, diminishing returns will hopefully mean this will be less of a problem moving forward.

Nintendo has only ever had one console superior to its competition - the SNES which put the TurboGrafx-16 to shame, and the Genesis, which only bested the SNES in processing capabilities. Even the NES lagged behind the Master System and 2800.

It doesnt matter if a game is an old ip or not as long as they add new elements or the gameplay feels fresh. For example Mario Sunshine, Mario Galaxy and Mario 3D World all play very differently despite being the same ip. On the other hand u have games like Call of Duty that add very minimal differences from one installment to the next.

You are certainly wrong about the handhelds.

The only reason you consider the Wii U a generation "behind" is because you are comparing it to Microsoft and Sony's platforms across the two generations. That's fine, whatever. Majority Rules, I suppose.

There is no such majority in the handheld domain, where there are only two combatants. One of these two is Nintendo, who has dominated the handheld market for 30 years and has never been outsold by any effort of their competitors. The other is Sony, whose current handheld is stalling at under 15% market share.

Nintendo's strategy with their handheld consoles has nothing to do with Sony's machines. Nintendo has been making handhelds since before Sony ever dreamt of entering the video game industry. To suggest that Nintendo waited to see (or guessed) what the Vita would be capable of, then consciously adjusted the 3DS to ensure it would be "one generation behind" that is laughable.

Nintendo's handhelds have consistently been roughly on par with their home consoles' capabilities two generations prior. The GameBoy, released shortly before the SNES, was less advanced than the NES, lacking even color visuals. The GameBoy Color was much more comparable to the NES, and released shortly after the N64. The GameBoy Advance was roughly on par with the SNES, and released shortly before the GameCube. The DS, released shortly before the Wii, was most comparable to the N64 in power. And the 3DS falls in the same range as the GameCube. It sounds as though their next handheld will be aiming for Wii U-like power, finally bucking the trend.