GC was a flop, 3DS is not. This isn't the Wii U we're talking about here. The installbase of the 3DS is big enough for Pokemon to sell at it's potential, so there'e no reason for extreme measures. Plus if Sun/Moon was also for NX, they wouldn't have announced it before the NX itself is revealed. But yeah fine, it isn't 100% confirmed, that wont be until the NX reveal, but it's a very safe bet it'll be 3DS only.
It's not about Pokemon selling at its potential. Its about making the NX a lot more attractive asap in it's lifetime.
Likewise it's not an extreme measure either offering the game on the cheaper 3DS while similtaneously offering it for the more expensive NX.
And I would argue that the announcement for now for 3DS is due in part to keep the 3DS going, sales wise, until the NX reveal (whereby 3DS sales will most likely begin to dip quite noticeably) and because there are other promotions taking place with Pokemon Sun/ Moon (like the upcoming movie reveal) which they need to work alongside. NX is just not ready to be announced now.
I could be wrong. But I am just being honest about what I feel Nintendo will do: anything released for 3DS/ WiiU around the time of NX is going to be on NX too.