I don't believe Nintendo would do well to wait for one of their consoles to sell poorly before making a change, I think they have been innovative enough as a company to get ahead of that curve, where you will have more than 10 years of the DS style handheld by the time the 3DS sees its end of life, as they did with the Gameboy, they could move on from that method, though it certainly removes the dual screen aspect from such a device if you take it with you.

I think we could certainly be heading that direction but to get the power necessary to display modern graphics on large HD screens into a small portable, lightweight device, that has battery power that lasts more than 4 and a half minutes might still be a bit of a stretch. The return to digital over optical storage though is something that could certainly pave the way for making a portable home console.

Also let's see how the the 3DS goes before we assume that mobile phones are taking over the gaming world. If Nintendo can still sell 100 million 3DSs in 5 years I can't see them giving up on handhelds just yet.

The Wiiu tablet is dependent on the console, and its fat because its a controller. It doesnt have the processing power or specs of dedicated tablets. So my answer is no.

My original point was not the Wii U GamePad itself, but the way its designed, and as an independent device to succeed the 3DS in the future.

It could be a redesign of the WiiU also, keep in mind. Mind you, the player holding the WiiU Portable will be receiving and sending a shitload of wireless signals. It wouldn't be really healthy :P