My thoughts on this...

1) Sometimes more isn't better, it's just more. And sometimes, more is worse in the big picture. The academic label lately is MRV - maximum recoverable volume. You might be able to do an extra couple sets of leg press today, but if that hampers your deadlift volume four days later, you're missing out on productive lifting. Yes, you can figure it out by tracking how you feel and perform as you tweak your volume... But a three month rookie who can barely find their ass with two hands and a spotter is going to get frustrated with crappy no-progress workouts and feeling beat down. Even if they could totally do twice as many leg press sets that day.

2) Pareto Principle aka 80/20 Principle aka diminishing returns. You get the largest portion of your progress (the "80%") just by showing up and increasing the demands on your body regularly (the "20%" of possible effort). Each smaller slice of the remaining 20% potential takes a doubling of effort again. You could work twice as hard as the guy who punches the clock, and progress another fraction faster than him. You could train as hard as you could eat, sleep, and recover from, then add a full diet and supplement regimen, then drugs, and end up a bit stronger, bigger, and leaner than him.

Meanwhile, he's putting in his hour and a half and heading home to see his fam, have a hobby, think about something other than lifting 24-7 like a chosen-at-birth Russian protege.

Anyway, yeah, I agree with your conclusions. Most people can be doing more and everyone should try to increase their work capacity. Not that everyone needs to, but if you're stuck, ummmm maybe try working a bit harder.

Nail on the fucking head.