That's exactly it though. He refused to lose those matches. Djokovic, Federer and Nadal had quite a few matches like that throughout their careers as well. Besides we don't really know what effect his Grand Slam win had on him mentally. He could either go on a tear, or maybe slump a bit. Or maybe it didn't change anything at all. Which one will it be? I feel your statement is premature. As are statements about the Federer/Nadal era being over.
Well, he was handed the match against Cilic on a plate (all he had to do with be there and put some balls in court) and the match against Berdych was a write off due to the wind. Berdych nearly sneaked it into a 5th set despite being a murderous wind player.
Going on a tear or into a slump is irrelevant 3 years from now. I think Del Potro's going to have his chance whether it's within the next few years or even further into the future. There's no one coming through and despite things being predictable right now with the top 4 dominating, it's hardly going to be the case in another 4 years time.