Only ranking the 6 best results actually hurts dominance as much as it hurts longevity. It would be fairer to continue to include results past the 6 mark but only if it continues to show a level of dominance and success that should indeed be rewarded and shouldn't go unnoticed, such as reaching the finals of a Slam Event or winning it only, and perhaps half points for any semi-finals but nothing more. This way you penalize longevity without sacrificing too many of the hard earned and excellent quality results that do occur.
At the moment you are even dismissing finals results that go past the 6 best results in the same manner you are dismissing a 3rd round result, which is surely unfair. There would not be many instances in which results would change if you were to make a change like this but I'm guessing their would be some, and in such a case you would be further highlighting their dominance and incredible high level play by only including semis, finals and wins past 6 rather than wins, while still not rewarding longevity so much.
Also, have you thought about extending this list to include the entirety of the Open Era, as before 1990 there was the Grand Prix Championship Series which basically is the Masters Series but with different branding.
Please do consider these things, they could form a new thread that works along similar lines and yield more comprehensive results, and you'd certainly be the person to put it together.