You cannot keep putting on muscle for ever. You can never get to Arnie size unless you are a freak of nature. Even if you train twice a day, didn't have to go to work, had perfect nutrition, perfect life (good mix of stress and happiness).Our bodies are just not designed to be like that. That's why genetics and "strongest survive" and dominant genes preside over that fact. Bean lean and "muscly" is more natural than beefy and lean. i.e. Greek and Roman soldiers were the epitome of "natural" athleticism in human beings, great diet, and not super bulky, but lean and strong.
Sure, we will evolve, and change over time as our genetic pool modifies itself and changes. However, if you look at a standard distribution curve 95% of people will fall in the middle, and I can guarantee that it'll only be the 2.5% that fall in the 1-2lbs a month category if that.
Yes you can keep stimulating , change diets, etc... However, there will be a balance between being lean and muscle mass that has to be taken into consideration.
Also in that SD curve, you have to take a sample of people at the same BF% and maintaining that BF% throughout the study, else it's called data cleansing/skewing.
If you're allowed to increase your BF% or decease it that SD curve is straight away useless and the measure is no longer valid. So we can look at all the studies available at the moment, none really can offer any definitive answers, however, it's clear that if we could all do it, there would be meatheads everywhere - there aren't. And the ones that are, get there artificially, or are genetically predisposed (some of the strongmen for example).