Prediction: The NX will not be backwards compatible

[MrCkurab]

While it was once the efficient architecture to go for, thats already long ago (by computer industry standards at least).
The base reason to use it was simply that, yes, it was much harder to optimize and really get all the power out of it, but the power you got out there was quite impressive, as it allowed you to get much more power out of your hardware than the competing architectures. In the older pre-HD days that was an advantage you just couldnt ignore.
But during the last generation and even more today we have games that arent pushing the tech to its limits on a regular basis, we have more power than we need, so why go the extra mile? Not to mention that X86 and ARM have evolved much more when compared to PowerPC, which, basically, is still as complicated to use as 5 to 10 years ago. It just didnt keep up, and likely never will now, as hardly any mainstream hardware uses it anymore.
On the other hand X86 and ARM easier to use and are already in use in a whole lot of hardware, meaning developers know how to work with them. Yes, on a technical level, PowerPC is still the more efficient architecture, but thats pretty much the only thing in favor of using it (Well and BC on NX).
Alone for third parties who are tired of Nintendos difficult to code for hardware a modern architecture would be a godsend. Remember all those games that were announced for Wii U just to be canceled in the years after, often together with degrading comments about how bad the hardware supposedly is? Those people simply didnt see the worth in completly getting into the architecture for one port, because without using the architecture to its full potential the Wii U simply was to weak. Its only because Nintendo knows how to tap into the consoles full potential that it is able to still make Games that graphically can somewhat keep up with the competition, regardless of the much weaker hardware.
Yet, in the end it would be better to give in to the pressure, but a bit more power in the box and get third party support and other advantages of a modern architecture over PowerPC.

tl,dr: nobody uses PowerPC, its peer pressure by the third-party developers!