First of all your not going to fry your CNS with intense, heavy, high intensity leg presses or squats when you apply the appropriate amount of days off after such an event. Secondly, controlling rep tempo is an excellent way to prevent joint damage from ballistic training, using 3-4 second negatives, with controlled turn arounds at the bottom. And you have to be kidding if you think the most massive quads ever built was because of leg extensions. That's a namby pamby movement compared to Squats, LPresses, and deadlifts. And the example I used with skull crushers was to illustrate it's not one of those one arm movements that I think are worthless. The same as choosing another "isolation" movement like barbell curls over DB conc curls. And back to leg extensions, the only value they have is using them in a pre exhaust fashion as explained to me by a well known BB in the 90s when I competed at the state level.
What I'm trying to show you is that there is nothing special about barbell compound movements that make them better for "mass," if so, what is that mechanism which isolations lack? You have to name it. Hint: you can't.
In regards to the CNS, if you didn't destroy yourself in the gym each day doing compounds that aren't necessary for mass, you would able to get in more sessions per week, and grow more times per year.
Leg extensions are not a namby pamby movement for quads; they are one of the only movements, along with the sissy squat, that loads the quads with 100% because the load is directly opposed to the quad muscle, and go precisely with the full ROM and strength curve; squats and leg presses do not. The reason why you can put 500lbs on a squat, but can barely do your own body weigh with a sissy squat, is because the sissy squat is actually harder, and you're putting MORE mechanical tension directly on the quads with less weight compared to the squat.
As bodybuilders, are job is to develop each muscle to their full potential; I highly doubt you can build a great physique just doing compounds; but you can build a good body doing just isolations, for body builder purposes, because there is nothing about isolations that prevent each muscle to get fully developed with them.
Squats and leg presses only have one advantage, and that's hitting multiple leg muscles at once time during one movement; however, for the very same reason, they have a big disadvantage. Squats for example, only load the quads with 30% of the weight, which means you need more weight to get the same amount of stimulus you'd get with leg extensions with less weight, and that's with loading your spine and fatiguing yourself using several muscles at once. Moreover, NONE of the leg muscles get a full ROM with the squat. NONE. Further more, during one half the movement, your quads are activated, and the other half they aren't, becuse they have to shut off (reciprocal innervation) in order for the other muscles to function and do their part in the movement. That is what I'm talking about when I more efficient and safer. Do not confuse an exercises full ROM with a muscles anatomical structure and its strength curve and purpose.
So it seems to me you are equating effort with stimulus, and that is a fallacy. Moreover, there are plenty of powerlifters who squat their face off with shitty leg development, that's due to genetics for sure, but also because they are not bodybuilding. Bodybuilders do squats and leg presses, but they also do a ton of other shit, all of that adds up. And again, just because an exercise may be less efficient and load the muscles less, doesn't mean doing enough of them won't create muscle growth; they will, but at what cost?
Now imagine doing this: instead wasting a ton of of energy on a tons of sets of compounds, you instead, fry the target muscle with more volume of exercises that literally are designed to destroy that muscle with 100% of the load. That is a smarter way to train, and will yield the same results if not better, without risk of injury and waste of effort.
Think of it like this, which would fry your triceps better without using an excess amount of energy or weights: heavy close grip bench, that fucks up your wrists, involves shoulders and chest to an extent, does not follow the true full ROM of the triceps, or doing cable press downs with a rope? The answer is the latter.
Same with barbell curls: you can load a barbell curl with more weight, but in reality, that's an illusion, because again, it is relative to the exercise. Standing dumbell curls actually are easier on the wrists, and stimulate the biceps even better because you're not locked in stiffly on a bar, reducing the full ROM, while also not taking into account of "bilateral deficit" (one side being stronger than the other).
full ROM is always superior to limited ROM.
So it seems to me that your position is that heavy and hard compound movements are better than isolations for mass, but that simply is not true, and there is no physics or anything to back that claim up.