Welcome to the board
The only argument I would raise to your post (and this is as devil's advocate) is that the stars you mention are all American. Federer is not--and with his quiet demeanor, is not the marketing star Madison Avenue would like.
Andy Roddick is much more of the would-be star, but he is quite failing at this right now, and so IMG is trying to get everyone to look at Donald Johnson.
Pete Sampras was the dominator in the 1990s, and he was not a marketing star, but still very popular among casual fans (notice that people who don't watch tennis now still refer to the days of Sampras, Agassi, Graf, and even Hingis).
What the advertisers really want is a brash star recalling the days of McEnroe and Connors. On the women's side, they are lucky to have a new blonde in Sharapova (although she isn't dominating which may become a hindrance) and a star with personality, Serena Williams.