Yes by Wii Mode litrally emulates a Wii and only because the hardware is practiclaly the same. To do a Wii U mode on NX you would need way more power as PowerPC arcitecutre is past it's life, or you have PowerPC bits on the console. If they stick with PowerPC then they will fall further behind the competition.
Also Wii Mode let's me play my physical games. What is the point of a Wii U only mode for digital games? For people like me that is a useless feature as I would need to buy the game again. If they want to capture a new audience, may as well just do what Sony did on PS4 and release some "remasters" of the popular games and you may even get some repurchases too.
"Only because the hardware is practically the same"
"Absorb the Wii Us architecture"
Anyway, to be frank, that's too bad for you. It doesn't matter that Wii Mode lets you play physical games. If the Disk drive wasn't on the Wii U, Wii mode would still be a BC feature, which is what we were debating. For people like you, it would be useless. That's too bad. Doubt you have absolutely zero digital games on your system, but who knows.
For everyone else, the library is all there. With BC, they don't need to do what MS and Sony had to do. Instead of wasting money, time, and resources remastering year old games, they'd have every Wii U game, retail, indie, AND VC, available on the eshop marketplace, which would be the NX's main marketplace, day one for purchase with the same exposure as any new game on the system would have. Then they could focus there resources on other content instead.