Hindsight is always 20/20. I honestly thought that Soderling would bag a few slam titles. Nope. Mono took him out. Going way back, everybody called Lendl a major choker. He was getting his butt kicked until he was 24 year old. By then, he should have had a bunch of slam titles. Nope. Zippo. Having 0 on your 24th birthday was a terrible sign back then, especially when guys like Wilander(4+ years younger than Lendl) already bagged 2 slam titles before Lendl bagged one.
But then, you have guys like Agassi. He lost about 18 months of his peak, due to meth and the downtime that the meth use caused. There was no way that he was getting to 8 slam titles, given the fact that he had fallen outside of the top-100 at age 27. Wrong again!
There are too many times where guys with immense talent fall short. But then, you get some guys that really step up. Looking back, Agassi's comeback from the ashes of death is one of the best stories out there. Lendl's and especially Djoker's improvement after they both fixed their diets at an old age is quite impressive as well. After the 1985 FO, the 20 3/4 year old Wilander had 4 slam titles. 25 1/4 year old Lendl had 1. And yet, Lendl passed him while setting a new record for weeks at #1. Djoker shattered every big record out there.
As I said, hindsight is always 20/20.