Del Potro will not win another Slam in his career. That's obvious.
I also think he will decline pretty early. About Masters 1000, the chances go like this IMO:
Won't win any Masters before retirement : 60%.
Will win 1 MS tourney before retirement: 39%.
Will win 2 or more MS tourneys before retirement: 1%.
There are right now 5 players who are clearly better than him. Year-end #6 for Delpo is kinda optimistic, but possible. Berdych should end this year as #6 IMO, unless he fucks up like the last month and a half. My prediction is that JMDP and JWT will fight for #7. Probably (and hopefully!) Del Potro will edge it.
Slams in 2013: 3 QFs and one 4th round exit.
If you don't mind me asking, who are those players?
And your prediction is extremely bold. Predicting the best player in the world under 25 years of age (by like a huge distance) has 0 chance to win another Slam? Either you know of some prodigy kid about to emerge, you believe the current top players will last far longer than usual or I dunno. But I seriously doubt you'd put your money where your mouth is anyway.