The game is much more physical than it has ever been it's hard for the younger guys to make the break through against the physically stronger and much more experienced top veterans. The game has changed.
DelPo was able to break through because he's a freak physically and able to consistently hit the ball hard enough to trouble the top players. It's harder for guys under 21 to win a slam than ever before.
I agree with your post except JMDP has had serious injures that were probably caused by the way he hit his big forehand.
I like some of the new generation and if people give them time, they will deliver. The game has changed dramatically in the last few years and barring a genetic freak, the days of teenagers winning GSs are over.