Nuh uh, don't try moving the goalposts. This argument was about the better defender/rebounder, not best basketball player.
http://www.nbadraft.net/players/chris-singleton
http://www.nbadraft.net/players/kawhi-leonard
Chris Singleton defensive rating: 10
Kawhi Leonard defensive rating: 7
A lot have said Chris Singleton has elite defensive talent. Yeah, the report on Leonard is he's a good defender, but you didn't hear anything predraft about Leonard being an elite lock down defender down the road.
I'd even say you're being very kind to Singleton by giving him a 6 offensively. He'll be an offensive liability until (assuming he ever does) develop a consistent shot defenses need to respect. He'll never be a threat off the dribble/in the post. Leonard is undoubtedly much better than Singleton with the ball in his hands, but he's not good enough where he'll be taking the ball out of Parker and Manu's hands.
I'd say Kawhi is the better overall basketball player, but IMO Singleton can have more of an impact without touching the ball.
This would be relevant if we're talking 5 years down the road. Since the Spurs' primary goal for the near future is putting a better supporting cast around the big 3 so they can contend again, it really doesn't matter that Kawhi is younger. If anything, it means he's gonna need more time, time Duncan and Manu don't have.