From using Google. It is said that at that time the new console namend NX will be released.And with that a new handheld is going to be released which could be used as a part of the NX.
Thats why for me 300 Pokemon would be enough....I wouldnt get a feeling of "finishing" the game when I didnt catch all Pokemon....but as you said for 700 Pokemon at the moment you would need a looooot of time....