I think if you look at who the players are that consistently make the semis of the major tournaments, it does seem to always be the top 4 players, but I think that is more a case of a gulf in talent between them and the next 4 or 5 players.
I think with Nadal being out, it does weaken the competition and you can more or less select the winner from the top 3. Federer is obviously getting older (arent we all), but until he came up against an on-form Murray, he rarely broke sweat in any game. Similar to Murray really.
And some of the tennis the top 3 play is quite brilliant really, so I dont think its a weak era as such, I just think they are so much better than the rest, they make it look weak.
"Tennis is just chess with balls....zugzwang"
"The older I get....the better I was"