Honestly, I highly doubt NX will not be backwards compatible, for these three main reasons.
1. With the lack of third party support and several droughts, there are at most, around a grand total of 20 Wii U games worth going back to.
2. Nintendo would have to keep the PowerPC architecture, which I don't see them doing.
3. Honestly, I really think Nintendo will do physical media like SD cards, considering some of the benefits it can provide over optical media.
"Just for comparison Uncharted 4 was 20x bigger than Splatoon 2. This shows the huge difference between Sony's first-party games and Nintendo's first-party games."