Yes but not impossible, as I can see that the only player who is able to stop Djokovic winning all the GS's in the next 3 to 5 years would be Murray. My point is never say never.
Never say never is one thing, but you're quite unable to use historic examples as foundation. If Djokovic pulls this off it will be quite unprecedented - 2/3 of slams after the age of 26. No other multi-slam winner has accomplished this or even been close to it in the open era. Most slams after 26 is Federer's six, which is a good bit less than half of his total even if he adds a couple more.
Seeing three years into the future is not easy. Noone knows what things look like then. Saying one player will win most slams in that period is presumptuous. Especially a player that has rarely if ever won a slam easy. It is always a cliffhanger with him - like Wawrinka this year, Nadal in AO 12, Federer in USO 11...do you really expect him to win every such cliffhanger the next five years? Then you are truly an optimist on his behalf. Nothing wrong with that, just don't get too disappointed if things doesn't turn out how you hoped. At this stage, remaining on six until retirement must be at least 20 times as likely as reaching 18. And I don't expect him to remain on six.