I don't think people are saying that GameCube and Nintendo 64 failed because they were powerful. I think people are merely saying that the power did not really help them be successful. I personally do not have the knowledge to claim that is the truth.
And in all honesty by specs, the Gamecube looked very inferior to both the Xbox and PS2 (which boasted unlit/untextured poly counts). In reality, graphically it punched above it's weight (sounds familiar), and compared well with Xbox, but in other areas it still lacked. For example, no real surround, small disks, less RAM, no HD support, poor network infrastructure (or plan). Nintendo had it's priorities, they wanted an efficient console, that could make them money from day one.
It was the most recent on par (or in the running) console relating to real world power.
As for could Nintendo had made a WiiU that's as powerful as the PS4 if they'd have left out the gamepad? Of course, but I think Sony is probably taking a larger loss than Nintendo is comfortable doing (given even with a powerful console, they still can't guarentee 3rd party support, or even consumer support without a gimmick), so it'd probably be priced higher.
For me, I wish'd they'd have snuck in at around Xbone power, skipped the gimmick (kinnect), and hopefully be able to sell it at $400, so I could get my nintendo franchises, but also have the best chance at 3rd party support...
Gamepad? Peripheral, I hate to say it, but given how all competing consoles now have 2ndary screens' available, I think 3rd parties will probably have alt support for these type of features. Given that most (even Nintendo) Wii U games support off TV play, it means that single screen experience is possible on those games, might as well make the gamepad its self optional as well.
Oi, why did we take such a side turn away from the GPU...