I don't think that's an issue that they care about for right now. Smash 4 isn't getting stale anytime within the next few years, and it has a healthy competitive community forming around it. It's going through some balance growing pains, but it's young. Also I take issue with the fact that a game with this many characters can get stale very quickly when it's being actively balanced and the base fighting mechanics click with a lot of people. I think Smash 4/Smash NX will have a long life. When it's time for it to be done, it'll go, but I think that's post-NX.
I think Heath is right, and we'll have a substantial update to Smash 4 sold to us as Smash NX, and Sakurai will get to skip making a whole new Smash for NX. It feels like Sakurai would find it expedient to just add more content to this game and let it ride for a generation rather than having to play the "who gets cut" game for another ground-up installment.
The fact of the matter is, Smash is being developed for NX right now, and has been for a bit (if sources are to be believed). If they want the game to have a healthy lifespan, they HAVE to convince the Wii U/3DS players to move to the NX version. And I don't think there's much of an effective way to do that outside of new content.