Well Canada, Cincinatti, Hamburg, Monte Carlo and Rome have been around for a very long time, before they became Super 9 events.
Starting at 1990 when it comes to a greatest player debate does leave out guys like Edberg, Becker and Wilander to a lesser extent, while not the greatest of all time definite greats.
It's just a question of semantics especially since Borg and Laver aren't included due to the timeframe.
As I said in the first post, I don't think it is possible to compare players of different eras. So what I did was take a point from where the atp calendar has been pretty much unaltered, which happened to be 1990 when the Super 9 format was introduced. No matter what starting point you take, players will always be left out. You can use this list as a testimony to the greatness of Edberg and Becker, though. Even though both were roughly halfway their careers at 1990, they still achieved more in the nineties than the likes of Kafelnikov, Chang and Ivanisevic.
In my opinion, this project a fair way of comparing players from 1990 onwards. I would applaud every effort to try and do the same for a longer timeframe, though.