I'm not saying he's not. I'm merely stating that I do believe Murray will suffer losses to lower ranked players at some point. Nobody would've been talking about Murray's new found mental fortitude if he had been beaten by Cilic like he should've been (his comeback had little to do with his mental strength, considering Cilic hardly put a ball in court). And if it wasn't windy, I seriously doubt he'd have got by Berdych, never mind win the final.
My point is that I think there will be an opening at some point in time for Del Potro, Murray's no Federer who was always guaranteed to win slams.
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.