Dolgopolov85
G.O.A.T.
Berdych has certainly declined. He used to be a constant fixture in the top 10. Now he is no.18 and hasn't been top 10 for more than a year. Tsonga has also declined. Both players used to be dangerous second tier players when they were younger. Now they are irrelevant.
As for Wawrinka and Delpo, here are my explanations:
Wawrinka never really had a prime before 2013. He was mostly a journeyman who couldn't put a dent in the dominance of the top players. He didn't and still doesn't have much mileage since he wasn't relevant in the biggest tournament. I doubt he would have still been as good in his later years if he had started winning big at a significantly younger age.
Delpo has had his prime years stolen by various injuries. So he also doesn't have much mileage on him since for many years he was sidelined.
With Berdych, I would say he still fairly gets up for the slams. Has lost to Fed in three of the last five slams failing which he might have gotten deeper in the draw. I don't disagree with your explanations for Wawrinka and Delpo but my point is we can't just look at players' age and say, "Oh, look, the young ones aren't able to dislodge oldies." It's not quite that simple. Again, if Agassi was able to win the AO at nearly 33, what's the big deal about Fedal's success? They are fitter and move better than him. I don't really agree with the reasoning that compares a Lendl being eased out with Fedal running the table in their 30s. How many past greats had to play a 14 slam champion in a slam final? Fed was in that situation that last year and it's entirely unprecedented. We have never had two players who both have surpassed the previous slam record playing in tandem. So it's going to take a lot to dislodge them. And since they are still playing brilliantly, why not just celebrate their brilliance while it lasts?