Hmmmmm. I largely agree, but in the spirit of detail as you mentioned, I subdivide the underachievers between those whose current situations are too comfortable/not painful enough, and those who, at some level, put in the work but sabotage it at some level. There is a small overlap.
I myself openly admit to being in BOTH of those groups as I sacrificed much training for my career (and would do so again); that was my choice in life. Even now I only just kicked up from 120gms to 300ms of protein, only just cleaned up my diet. Only just cut out chugging down 5-6 beers EVERY weekend night.
All of those are residuals of the two decades I dedicated to my careeer; the irony is that I found it easier to put the sweat in at the gym than to research a good diet. And I'm coming into a boatload of cash next year, so the career, as educational as it was, could have been replaced with time researching my diet.
There you have it. But I DO feel sorry for scrawny guys who can't stomach that 6th load of chicken.