Prone to losing to an in-form player? You mean an in-form top 4 player? Because he made at least the semis in 7 of the last 8 slams. The other showing was a QF loss to Ferrer on clay, hardly a surprise. He is also more likely to beat the 'top 3' than any other player, including del Potro.
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.