First Nadal is number 1 in the Race by far and he can become number 2 in the short term, but to become number 3, you will have to wait more, maybe far more.
Personally I'm more sad that I get old myself
But you should not really be sad of getting old : that's a natural process of life
, the feeling is more complicated.
Anyway, I don't think you should really be sad of getting old.
It's rather sad that you get injured (Nadal)
or that you realize that you missed by little an important achievement for you (the calendar slam -twice- for Federer).
Generally speaking, I mean you cannot be sad that a 2-player rivalry may become a 4 (and soon 5) player-rivalry.
But you can be sad that you realize that two of these players might not be at their top anymore except for short periods.
So far, in the Race, Nadal is number 1 by far, Federer is number 2 by far.
And the others are chasing : their hope is that the first two may not be at their top anymore (at least for Federer it's certain).
And that they get better themselves of course.
For the answer, I think Murray or Djokovic will be number 1 before next year.
But anyway, the future between these 5 (add Del Potro) is more difficult to predict than the past.