Once Djokovic and Federer drop-off, who will challenge Nadal on clay? Murray? Nadal has a severe stranglehold over Murray at the slams of late. Nadal beat Murray in all 3 of their slam meetings in 2011. That is crazy. That's not to say Murray can't beat Nadal at hardcourt slams, as we already saw he did back in 2010 and 2008. But Roland Garros is the last slam Murray would be able to hurt Nadal at. Even winning a set will be a struggle for Murray at Roland Garros. So I don't see him doing anything that Federer and Djokovic couldn't do.
Even in 2012, Nadal most injury prone year, Nadal won Monte Carlo in straight sets, Barcelona in straight sets, Rome in straight sets, and only dropped one set at Roland Garros. So when Djokovic drops off, and Federer drops off, who is there to take a set from Nadal? Tomic's worst surface is clay. Raonic's worst surface is clay. It is surreal how weak the 2013-2017 clay era will be. On top of that, Nadal has said he will focus on clay now, and avoid some of the hardcourt events. He's increased his claycourt schedule which now begins in February. You thought Nadal dominated 2005-2012.....that was only the beginning.