Anything is possible... BUT he is the same age as Nole so the chances are their careers will finish at roughly the same time, Rafa is also only one year older than them both. The latter may disappear a bit earlier when his knees finally make it too difficult to compete at the level he is used to. So once Fed has gone in 2 more years, Murray has perhaps another 2 years of Rafa and Nole ,and then a couple of years of just him and his Serbian pal. Pretty much touching 30 that means he will need to be as good as he is now, and hope that the next best things to hit the sport don't become what any of the top 3 are now, as that would mean they would likely have the edge over someone in the twilight of their career. Then there is the likes of Del Potro who has already won a slam and can do it again when he get's back into top form and away from injury (plus he has 2 years on Murray), Tsonga will still be about too.
He makes it to the semis now because he is the best of the rest, on all surfaces other than clay where he isn't even that. There are players still around like Roddick and Ferrero who actually won slams early on their career and made multiple slam finals too, but never repeated. When Roddick reached the Wimby final 3-4 years back he was still ranked lower than Murray etc.. and came so close, it is not as if Murray has even won a set in any of his slam finals.
You have to think that if Murray is going to do it, it is going to be when he is at his physical peak which is now and the next couple of years, but barring injuries to other players to open up the draws for him, it is simply not going to happen. A lobotomy may also help to deal with the mental issues that clearly hinder his performances, if not for them perhaps he could closer to those currently ahead of him.