I think Tony has 2 great seasons left, then it'll de a quick downhill slope from there imo. As the above poster stated, comparing him to Nash is silly. Nash is one of the greatest shooters in NBA history aling with one of the greatest passers. Tony's game is predicated off of his speed and quickness and ability to get in the lane.
Plus, Tony takes a beating during the season. He's on the floor almost every time he drives to the bucket. That takes a toll on your body over time. As he gets older he'll likely slow down considerably and try to rely more on his jump shot.