I can't remember who was calling JWT's match today, but he made a good point: that Tsonga simply doesn't have the all-around skills (particularly defensive) as the top four guys. The top four really are exceptional in their ability to play both defense and offense with power and agility. Yes, they have great mental skills, but athletically they're also in another league.
I think Nishikori, with his extraordinary Agassi-like ball-striking, has/had the best combination of skills to compete, but he can't stay healthy. I also think Dimitrov may come through (although I do NOT see particular similarities to Fed). I'm not convinced Raonic can ever break through unless he lucks into a really thin field in the coming years, a la Krajicek, Stich or Ivanisevic. Yes, he's got the great serve, but from the ground he's no Juan Martin del Potro.