And? He was also routinely getting taken to the cleaners by post-prime Federer at Slams and only managed to eek out a Slam win over him a few days ago when, by any unbiased observation, Federer isn't even remotely close to the heights he reached in his prime.
Murray is much luckier with the era he peaked at than Safin and Hewitt were. All the rivals Murray has to face put together are not such an iron walls to the Slam gates as a prime Federer was.
Is 28 years old "post-prime"? I'm sorry, but that's nonsense. No reasonable person who saw the 2008 USO final and 2010 AO final could say that Federer wasn't peaking in those matches. He was phenomenal in both.