For the sake of debate, two problems with your statement...
1) at the prices I was talking in a fully restored zero mile drivetrain truck, you don't need to run a smogger 454. You can use a modern engine/trans and get much improved, likely comparable mileage. (Also, cucv trucks, particularly the m1009s, got 20s mpgs with removable tops using flawed 1980s technology.) Add in a double OD unit for a couple grand, I'd bet you get better mileage out of thee built truck than the bought one.
2) plenty of square bodies were rated to tow more than 6000#. Heck, plenty of square body trucks had payload over 6000#.
I've driven plenty of late model trucks. Even owned f150 4x4s new in '18. Comparatively, the squares drive better and are more comfortable, plus they don't dent if you look at them funny. It's not nostalgia. It's about capability, comfort, and mechanical simplicity/reliability.