Except we know there are other ways to provide bc. And not just including older hardware, The 360 came out well before the Wii u.
Nintendo was very obviously considering other ways before they were pleasantly surprised by 'things we would have never thought of' for said partners. It wasn't the driving force of the design, it was a pleasant surprise they worked out during the process.
I find a contradiction in 'They showed us something we never would have thought of doing!' with 'This was the main reason they made the console this way.'
Ah. Let me see if I can rephrase. The Wii U is built around the same CPU as the Wii and GameCube because it was the
most efficient way to get Backwards Compatibility. If Big N wanted As Much Power As Possible, Damn Efficiency, AND backwards compat, they probably would have gone with a different solution. If their goal was As Much Power As Possible, As Power Efficient As Possible, they might have gone with a different solution.
The "surprise" is that the designers figured out how to use the Wii backwards compatibility hardware for Wii U's purposes, limiting how much hardware they had to stick in the device. To quote the Interview
There were times when you would usually just incorporate both the Wii U and Wii circuits, like 1+1. But instead of just adding like that, they adjusted the new parts added to Wii U so they could be used for Wii as well.
In other words, the default design was "Stick a whole Wii in there for backwards compatbility." But they didn't they decided to share hardware. And we know they're not referring to anything other than the MCM because the next lines of the interview are
Iwata
And that made the semiconductor smaller.
Shiota
Right. What's more, power consumption fell. That was an idea that only designers familiar with Wii could have put forth. We were able to make such a small semiconductor because so much wisdom bubbled up!
If you look at the MCM there is only one part that is shared between Wii games and Wii U games. That's the CPU cores. Which goes back to what Hermii said about why they kept the CPU for three consoles
Was that their only choice? No, they could have gone with a new CPU for the Wii U and included a PowerPC pair for Wii BC. Again:
There were times when you would usually just incorporate both the Wii U and Wii circuits, like 1+1.
But the chip designers convinced them that they could more efficiently build the WiiU around the same CPU.
They
did decide to include a new GPU that was
not BC, and the solution there was to include a backwards compatible GPU but make it part of the MMC instead of putting it in as a separate piece of hardware.
TL;DR, the Wii U shares the CPU with the Wii and the GameCube because it was the most efficient way of achieving Backwards Compatibility.