I agree that if it were very cheap, it might not be a bad idea to add that BC.
But the problem would still be the GamePad. It's the standard controller on Wii U, so first, for the people who don't want to buy it for their new NX, Nintendo will have to explain, "okay, these Wii U games will work with your NX, but not these ones". It's a confusing message. "Backwards compatible with Wii U!* *Well, sorta, mostly, I guess". I think if Nintendo clearly understands everything that went wrong with Wii U, they'll avoid any confusing messages at all with NX.
My other point is that, if the GamePad is instead required for some hypothetical Wii U backward compatibility, and it's sold separately, people just won't buy it. I presume it'll be at least $80 because of the hardware inside it. If you ask people to buy a brand new ~$200 NX + a $80~100 GamePad + old Wii U games, they're going to balk. At that point, Nintendo might as well skip the BC entirely and just make a new Mario Kart, a new 3D Mario, etc. As long as they designed their Wii U engine(s) and created their assets properly, they might not even have to do too much work compared to what they presumably had to do to get those kinds games out on Wii U initially.