I'm living proof of it. But the enetic cap is very real -- both natural and chemically assisted. Since I trained a lifetime naturally I know my genetic limit was only around 175. Sure, I could have gotten bigger, but not without adding more fat. (Yeah, yeah, I know. I could have bulked and then lost the fat, which I did, but fat loss too has a genetic cap. I'd always wind up at 175). Steroids got me over 200, and years after stopping I managed a solid 195 without an increase in fat. Today, on HRT I stay at 190. I actually prefer 185 and more cut, but it requires a constant battle with my genetic limit, and at this point, age is a factor too.
Those numbers may not seem that drastic but if you imagine what wenty pounds of lean meat lookks like, it's quite a bit.
Now can you hold on to 30-40 pounds of chemically grown muscle without any help? Doubtful, but possible -- again, depending on genetics. Even still, that full, vascular, thin skinned look you get while on can not be maintained. You just have to look at that as a temporary gift. Of course, some guys live for that look and can't accept being without it so they make all sorts of excuses for not "really" coming off. ("Bridging, slin, HGH, HCG, a little var, a little clen, etc, etc, etc.)