I don't look at it like old trucks are replacing new trucks, I look at it like they're one of the last "vintage" vehicles that was affordable, so they took off with the younger crowd. If you look around, engine swaps are rampant...and frankly piss me off. I see it like this, the common hotrodding vehicles all were cheap and accessible (32-34 Fords, Tri 5 Chevies, Fox Body Mustangs), and many times engine swaps were done to use the cheapest possible combo. With everything down to a 6cyl Maverick now commanding $$$, the only older vehicles left seemed to be the shortbed pickups. The LS swapped "bumpside" is the new common hotrod platform.
Vast majority of these trucks aren't really being used as trucks, they're lowered considerably and many times the bed has to be cut to accommodate the frame kickup over the rear wheel.