Your point is based on wild speculation, so I feel free to speculate as well.
1) If Nadal had to retire at 28 he would had remained 11 years as an elite tennis player. He could feel very well satisfied, with or without knee issues.
2) At the age of 22 Nadal had already accomplished everything that can be accomplished in men's professional tennis: several slams on 3 different surfaces, a bunch of M1000, several Davis Cups, Olympics Gold in singles and world #1.
3) Apart from that, he will always be remembered as Federer's nemesis, the one that prevented him to increase his legacy, the one that won 2/3 matches against him, the one that defeated him in 3/4 slam finals (including surfaces like hard and grass where Federer is considerered one of the greatest ever to play on them). He will always be remembered as the dominator between the two in his rivalry against probably the best player to ever play the game.
4) He is not the pigeon of any other player, and he has not a differential of -2 in his H2H against any other player. He is probably one of the toughest players to beat ever and probably the best ever at the mental department. His defense and movement while fit and healthy is also unparalleled. He also has the best records while playing 5-setters.
5) That facts alone are impressive, but he also possesses the best records, stats and streaks ever achieved on clay. He is an extremely great player overall but has also mastered to near perfection his game on clay (he is considered the best ever on the surface by many).
Now let's speculate:
6) If Nadal wins another Wimbledon (quite likely to happen this year), 1 USO and two more RG he'd have 11 Slams, including 7 RG (the absolute clay GOAT forever), 2 Wimbledons, 1 AO and 1 USO. That means he would complete the Career Golden Slam including at least a couple of slams on every surface. Has any other player of the Open Era ever achieved it? Not even Federer.
7) If Nadal wins two more M1000 he would have 20. None of the current pro players are going to win more than 10-12 (at their very best) overall.
I don't need him to be considered the GOAT (and he won't) and I'm perfectly fine with his current career (it's TOP-10 right now), but if he manages to win 1 Wimbly (this year he has a very good chance), 1 USO (I have the feeling he'll have a shot this year or in 2011), 2 RG (HEAVY, and I mean HEAVY favourite) and 2 M1000 (quite likely to happen in 2010) then he'd have one of the best overall careers including the Golden Slam (not to forget he achieved the Olympic Gold on a fast hardcourt) with 2+ of slams on every surface (one-dimensional dirt moonballer, anyone?), 20 M1000, at least 3 Davis Cups, clayGOAT status for ages and the edge of the rivalry between him and Federer.
PS. Federer is great and 16>7. I'm just pointing that Nadal has nothing left to prove in tennis and he could well retire today and be remembered as a legend in the game (the same goes for Federer, obviously).
Wow, that's a surprisingly good, and even somewhat unbiased post from you. Well done.