To be fair, his was a downfall but it only looked so start when viewed in the context of the traditional career of a footballer, which is generally thought to be anywhere from 17-35 ish. Perhaps we need to expand that idea to take account of players who, in a game which is becoming more and more physical, peak earlier and are no longer able to put in those sorts of performances at stages in their careers that we traditionally think of as being their 'peak years'.
Maybe it's the case that for some players who, like Ronaldinho, base a lot of what they do around explosiveness and the ability to be a split second quicker than the players around them, we have to realise that there may be an earlier fall off in their productivity (for whatever reasons) and judge their careers accordingly. When you look at someone like George Best it's impossible to argue that he didn't have a wonderful career, despite the fact that everyone was left wanting more. It seems to me that for some players, there will be factors that conspire against them (though will often be self-inflicted), whether they be physiological or mental, and that has to be taken account of too. Personally with the way modern football is going, and the physical and mental demands it puts on players, I can see quite a few more players burning out of peaking much earlier that what is considered the norm.