Bit of an odd call that. Maybe not in the not-too-distant future but you can't rule him out a few years down the line. He's got the ability, both technically, and most vitally, mentally - as he proved in US 2009. Definitely capable of winning more slams, but a while 'til he's back at his best. Look at the likes of Ferrer, Tipsarevic, Fish, etc - playing their best tennis in their late 30's. If they can make improvements like that there's no reason Del Potro can't reach and and go further than his standard in 2009.
I'll be surprised if he never wins a slam, EVER again.
I would never rule him out from winning another slam, but I think the problem with him isn't so much that he needs to get back to his best, but that he lost a full year of development. Back in 2009 he was competing with the top four, but all four of them are much better now than they were in 2009. Tennis players constantly adjust and improve and even Federer is a much better player now than he was 3 years ago (despite the extra age). Del Potro doesn't just need to get back to where he was in 2009, he needs to get to where Djokovic, Nadal, Federer and Murray are in 2012.
That's his big problem and it's something that I think he might never recover from. When you lose a full year of development at such a key age it isn't just a question of becoming fit again. You can't get that year back and I think it's something which could limit his potential for the rest of his career.