People have argued about reasons for the delayed (and impending) arrival of the next-gen players: tennis is too physical these days, big 4 are too good, next-gen players are just not good enough, surface homogenization etc. The current situation:
Rafa, Nole, Andy and Fed (2013 aside) are still going strong.
Jerzy, Milos, Grigor, Tomic are still trying to make their name.
Khachanov, Kyrgios, Kokkinakis, Vesely, Quinzi, Garin, Coric are taking baby steps.
Do you think we are seeing a lost generation of tennis talent with Kyrgios, Coric and co. ready to take over by the time Big 4 leave the game or will the intermediate generation get their time under the sun?
Krgios, Coric and co ready to take over? What makes you think they're ready to take over? They've done nothing. Absolutely nothing. Coric hasn't even one a match at ATP tour level and you entertaining the thought of them being ready just in time to take over from the big flipping 4? Are you serious bro? There's not even that much hype surrounding these 2 player's, so what makes you believe they'll be successful at atp tour level. Kriogos has some weapons, but it's still too early to say anything, we have to see how his backhand comes on, otherwise he'll be exposed on that wing like Tsonga has been all his career. Coric doesn't have any weapons as of yet but is still developing. I am astounded you can suggest these are the player's who'll be ready to step in the moment the top 4 are hitting 32+. Genuinely astounded.
Anyway. The reason isn't only fitness but I believe it's years of "priceless, uncoachable and unattainable" experience on the tour and learning at the very most prestigous level. By prestigous I mean the latter parts of grandslams, those experiences you pick up from semi finals and finals at grandslams and masters events are simply priceless and something all parts of the top 4 picked up at a fairly young age. This is the reaosn I feel Dimitrov has not blossomed into the player all the media and everybody felt he would do, because he hasn't been "amerced" in high intensity, high tension, edge of your seat, in the heat of strenuous battle moments. Without that experience you can see he lacks that extra matchplay edge. He's not a good matchplayer.
As for Raonic and Jerzy. Well again, I'm confused. Raonic has only really ever recently improved his return of serve to even be considered to be a genuine, real prospect and Jerzy broke onto the scene as a late bloomer reaching the masters final in Paris. Since he's reached a Wimbledon semi final. So I'm not quite sure what you're expecting of him? Big ballstrikers like Raonic and Jerzy are always going to find it rather difficult to breakthrough and enter the top 8 player's in the world. You need a rallyball to be a consistent all-year around player, they lack that, particularly Jerzy. They lack a distinct amount of movement which they try to compensate for with their long reach. Their games are based around being "hot" if they're hot they can compete with the big boys, but if they're "cold" they are in trouble. That is essentially the difference, these player's whom have the great movement combined with weapons in their game can still come through matches where they may not be particularly doing what they do best. The likes of Jerzy Janowicz partiuclarly as an example, simply cannot do that, the bloke relies on clutch big serving on second serves, low margin for errors forehands that clean the lines. It's just not realistic to see this guy competing with the top 4 guys on a consistent basis because he does not have the movement and thus will need to keep a lowmargin for error game up continuously and that is bloody hard.