I still think he'll win a major at some point, but the issue is that the younger generation of superstars must be coming through in the next few years, so it may only get harder once Federer starts to dissapear.
I would have agreed maybe 3-4 years ago, that if he hadn't won by the age of 24-25 he'll struggle, he's only got the US and AO before he hits 26. the thing is though, there isn't a new generation. It's actually something slightly worrying for mens tennis as in another 4 years the quality could have dipped dramatically.
right now the young players on the scene are raonic, del potro, tomic, cilic, dolgopov, nishikori and dimitrov.
all these guys are 21-23 and would be expecting to make up the new gen but most of them haven't even cracked the top 20 yet (murray and djoko were top 10, i think, by the age of 19, nadal about 17 or 18)
del potro looks the closest to a dominant force, purely based on his earlier form though, tomic maaaaybe, but those two and most of the others are all just huge monsters that slug hard. only dolgopov and dimitrov look like traditional tennis players but they're certainly old enough that they should have been making inroads in big tournaments. what happened to donald young and ryan harrison?
add to this the major threat to big three beyond murray seems to be from players peaking late in their career like tsonga, ferrer and soderling. so in short, either there is a massive talent vacum behind nadal, djokovic and murray or modern fitness regimes and training are meaning it's taking longer to hit your best form.
I don't know, but murray certainly doesn't need to be looking over his shoulder for another couple of years because there's just no one there.