A creatures structure can only support so much. I think that as you push your bodies growth potential to its limits, you start to see a diminishing return for the amount of fuel required versus what you get out of it. Dante of DC fame seems to think that it's just because people change their workout style and dieting to conform to what they think they are supposed to be doing at that stage. And there is probably some truth to that. And some of it is that they're not seeking the maximal growth possible, but a controlled growth in only certain areas of their body. And they may have just pushed their body as far as it will go. On the other hand, it may be possible to find out what it is that is limiting them in their growth at that stage and neutralize it, whether with a different protocol or supplementation/"sports technology".
I think we do have some basic principles of hypertrophy. You break down the muscle and build it up. You fuel your body with protein. The more intense, the better your results. Get enough rest to let your body recover. Every workout has worked wonders for someone and totally failed someone. I don't think that the specific details of a workout program are as important as the work you put and the intensity you bring. Whether you are making a workout intense by doing lots of rep, lots of sets, heavy weights or forced reps or rest-pause or whatever, it is that intensity, that will to push your body beyond muscular failure that counts. Arnold always said that it was those last few reps that count, those ones that you can only get when you push through the pain barrier. In any style of workout, that is always the goal, to push through pain and failure.
Of course there is more to learn about how the body grows and how to optimize that growth. It's a continueing never ending process. But no matter what you're eating or what training program you're doing or what kind of supplements or drugs you're taking, none of it will matter if you cannot push through that pain barrier in your workouts and get a truly intense workout. Many people have found the way that best allows them to do that. And by following in their footsteps, you may discover the way that allows you to do that as well. Or you might not, and so you try something else. And once you do find what works for you, then you can apply that to every workout, to every training regimen. Arnold and Sergio both got big no matter how they were training, no matter what style, because they could bring their focus and willpower to bear every workout.
I love learning more about bodybuilding and the physiology and science behind it. I try to apply everything I learn, and then learn how to make it work best for me. But without the ability to apply that knowledge to ones own body, it does nothing. And I think that that ability is one that is learned through experience, through trial and error. If there were ever "True" principles to hypertrophy, I think they would be knowledge, instinct and will.