"Genetics" = A constitution capable of withstanding massive amounts of drugs. Of course there's no magic drug, it's just high dosages all of the time.
..Some people burn out or suffer health problems before they get to "that level". That's all. Those are the folks with "poor genetics."
I partially agree with what you say. However it is much more than simply being able to "withstand" the drugs. It's
in no particular order
1. Your genetic shape. Your structure if you will.
2. Your ability to lose fat evenly and still maintain your muscle mass. Some people's body's will just not keep a lot of muscle mass no matter how many drugs they take when they lose too much bodyfat.
3. Your response to training and drugs. Generally those that respond very well to training will also respond extra well to the drugs.
4. Your ability to dedicate yourself and keep an extremely consistent workout schedule and eating schedule over a long period of time.
5. Your ability to withstand the drugs over a long period of time.
6. Luck.