If the NX goes Arm route, and go mobile having ps3+ performance, then it will ve very close to high end smartphones and tablets. Todays Smartphones and tablets have graphic cards that have ps3 performance. Two years from now, they will be even faster. Casual gamers will play on that platforms, not in NX. Maybe japanese will. Hardcore gamers will be on MSft or Sony side.
If those same hard core gamers are so concerned with power why don't they just go with a PC. Hell, hardware is cheap enough that you could make a $500-600 computer and get the superior experience. I never understood that about the people who ragged on the Wii U's lack of power.
I would also mention that steam machines are also in Sony and Microsoft's rear view mirror. It's not an immediate threat, but eventually more games will be compatible, the operating system will be improved, and with the right price cuts, they will begin to increasingly siphon off sales long term from both Microsoft and Sony. The console market is going to be very crowded.