I don't have any figures to back this up, it's just a gut feeling, so please feel free to shoot me down.
I think it has something to do with the change in the distribution of ranking points. Established players get higher points from slams and MS so one or two good performances keep them in the top 100. More difficult for new (=younger)players to break through playing lower ranked ATP and challengers.
not at all.
Look at Zeballos, Lorenzi and a few others : you will see that it's not easy at all to survive at the top when you always have to play top-tournaments rather tahn many challengers like previous year.
I disagree in the importance of last changes about that (challengers have not been really disadvantaged comparing to ATP250 tournaments) and I rather think that challengers are still advantaged in points comparing to main tour tournaments.
And anyway as far as the topic here is concerned, people have noticed here that one main problem is that very young generations (from 1988 apart from the best 4 of that generation to the later years) don't succeed at all.
And they don't even succeed in challengers and even futures ... except 1992 generation (Tomic, Harrison, Krajinovic, Bhambri, Basilashvili ...).
Anyway, for very young talented players, very often they are not regular and endurant and rather reach the top by a few big results (esp many young great talents revealed at slams).
They are not endurant and regular enough to accumulate many big results in challengers like Zeballos, Lorenzi, Devilder, Ventura, Ramirez-Hidalgo, Kim, previous years even Reynolds or Kendrick, Daniel.
Among youngsters, only Dolgopolov (21 years old already) has done it recently, but he couldn't improve his ranking like previous players because he played half less tournaments ! Pere Riba (same age) plays many tournaments but with little success.
Then I don't agree at all with this reason, and God knows that I do follow the rankings
From the generations point of view, the generations after 1981-1982 were already less good than the previous ones (except generation, 1987 which is quite good for the number of good players), but generations 1988 to 1991 look even worse
Contrary to what many people say I also think that generation 1981 (Fed-Hewitt-Nalbandian-Davydenko ...) was great : usually every generation has 10 players who reach the top-50, 1981 generation had around 20 of them !
And generations 1980-1982, even 1979 now (Blake-Ljubicic-Stepanek-Gaudio) are also still there.
As for the reasons, there are probably real technical reasons (the game esp is slower and more tactical) and also a generation gap for a few generations.
In the past, many big young generations emerged at the top when there was a change in the game, for instance the new rackets in the end of the 80s (many youngsters playing with a lot of spin like Carlsson, Perez-Roldan or Davin, later Bruguera or Berasategui, rather than attacking players in the beginning), beginning of the 90s (players hitting very hard like Courier-Agassi and players with a big serve like Sampras-Ivanisevic-Krajicek), the game maybe slowing down when Fed-Hewitt's generation emerged. In their time, Borg's generation had also brought a big physical dimension.
This time there's maybe too little change
Maybe it will change : from last year we have seen more players very tall and hitting very hard succeeding ... but still the game is so slow that it's not easy for them : see for instance Korolev and Gulbis, they have to add other tactical elements (as Del Potro, Söderling and Cilic did).
Tomic and Krajinovic are tall players, but you can see, even Tomic chose a more tactical game than hitting very hard.