Just talk to anyone who trains endurance sports. Read the recent "how to skate 10kms" that is making rounds lately. Look at any training program of competitive endurance athletes and you will see low intensity volume before anything else. The reason is there is that it works. There are adaptations that only happens during long easy sessions.
If you just care about carrying heavy stuff then you can ignore all of the above but there is more to fitness than that.
>>All other factors being equal, the man who can squat 400 lbs will always be fitter than the man who plays VR for 45 minutes every day--and the former can do it with a smaller time investment
Plenty of guys who can squat 400lbs who would be totally gassed after running a few miles or cycling up a moderate length hill. I wouldn't call it fit. I am betting on 45mins per day VR guy.