The long and short of it is that if they tack it on to the end of the 3DS' lifespan it would end up a filler generation that pushes back the transition from the 3DS to its successor. Look at 5th gen. BW1 launched at the tail end of the DS' lifespan, in fact internationally it launched AFTER THE 3DS' LAUNCH. Then we got BW2 about a year and a half later, and then went straight onto 6th gen in 2013. Not only was 5th gen cut short because it was released on dying hardware, but it also cut pretty far into the 3DS' lifespan before we actually saw a 3DS game. Furthermore, being stuck on current gen hardware limits what kinds of changes could be made to the game. 5th gen improved very little on 4th gen, most of the actual changes in the game stemming from a change in design philosophy that represented a step sideways or even a step backwards as opposed to a step forwards. And in 6th gen's case, there is one rather significant flaw in the hardware that I would not like them to prolong for another 5 years on the 3DS, and that's the lack of a controllable camera (which would require a second Circle Pad, which most likely won't happen until the next gen handheld). So, it would probably be a glorified Gen 6.5, adding very little besides new Pokemon and wasting everyone's time while they wait for a true next generation on the next handheld.