2008-2010 wasn't post prime, it was just Federer coming back to Earth after 4 years of stratospheric play. Not as consistently great as he was before, but still more than good enough to handle anyone apart from Nadal at Slams. It was in 2010 that he had a noticeable physical decline.
The point is though that you make it sound as if Hewitt and Safin were luckier than Murray with the era they peaked in, which is beyond absurd. If Murray's peak had coincided with Federer's like theirs did, odds are Murray would have never even sniffed a Slam.
They were luckier because the top of the game was not as strong.
OK, let's look at this qualitatively. Name some things that Hewitt, in his prime, did better than Murray.
Safin is much harder to compare, since he was so much more erratic.