It's one of them:
- The processor is the same tech (PowerPC 4xx from 1998 just like the Gamecube and Wii), just with a higher clock speed and tripled the cores.
- Ati GPU in all those models
- Can use all the peripherals of the Wii out of the box
- can play all the games of the Wii
- The console even looks similar, especially from the front; Wii U case is basically just a streched Wii case with rounded edges.
From a technical point of view, the Wii U is indeed just a Wii HD with a Gamepad controller slapped on top. From a marketing point of view, it's the successor of the Wii.
I am a bit interested in how a Wii HD with Motion Plus would have done if it released in 2009/2010.
I think it could have slowed the decline down in those years and extended lifetime sales to 110-115 million or so.
Probably, especially if the Wii HD would have had similar specs to the Wii U, as it would have allowed easy porting of PS360 games. But by how much, that's more of a question to me. While it probably would have slowed down the sales downward trend, I don't think it could have reverted it, So 120-130M would be maximum sales for a Wii with an HD mid-life update.