They'll be there abouts for another few years. Who else is going to overtake them?
Who knows? There's a bunch of underperforming guys in their early to mid 20s who might eventually get their shit together. Dimitrov has been spudding it up for a long time, but the elements are there if he can get an attitude transplant and pull it all together. Cilic and Raonic have potential even though they seem to be mostly wasting it. Tomic may break through. Look at Federer - one day he was spudding along as a top 15 player and everyone said he was an overhyped pretty boy. Nek minnit, he's winning Wimbledon.
It doesn't necessarily have to involve them becoming as good as Djokovic and Murray are now either. I think people are forgetting how quickly defence-heavy players go from slam winners to highly beatable spuds as soon as they lose a step of speed. Look at Hewitt, Chang, Wilander. Murray and Djokovic have good weapons but if they can't get into the right position with time to hit them comfortably they suddenly become very ineffective. They're only an injury away from that happening. If they drop back a bit, suddenly they may find themselves losing to guys like Tsonga, Berdych and Del Potro who are less able but play games with likely greater longevity.
I think that unlike Nadal, Murray and Djokovic have the ability to adjust their games to their aging bodies - I actually think Djokovic could have great longevity if he moulds himself into an Agassi-style player based around his return and offensively dictating rallies from the middle of the court. But it remains to be seen if they in fact do that.
Djokovic and Murray may rule the game until they're 30. But I really don't think they will.