Until the price and release date of the NGP is announced I don't think much can be said about the viability of the NGP ...
Essentially, if the price is high enough, the release date is late enough, and the 3DS is popular enough, after the NGP is on the market for a year the 3DS could be selling 3 times as many systems on a monthly basis and have a userbase which is 5 to 6 times as large; and when you couple this with the NGP having much higher cost to develop games, with much longer development timelines, it would be very difficult for any publisher to justify developing games for the NGP.
In contrast, if Sony somehow releases the NGP at near to the price of the 3DS, and is able to release in the not too distant future, both systems could be viable and the generation could be much closer than the previous one.
