While it is perhaps true the F-150 is rated to carry payloads comparable to those of older 3/4 ton's, it is debatable whether the F-150's mechanical hardware is just as durable as that of the older HD's.
The F-150 frame, axles and overall chassis are somewhat tame compared to what you find underneath a 10-year old Ford or GM HD.
Bigger does not always equal better. Look in to the quality of the metal in an older F250 frame, now go do the same for a 2018 F150. 20.00 says the new F150 frame is built out of a higher strength steel.
You also are not taking into account that it's a fully boxed frame that is light years ahead of the older trucks when it comes to design and engineering.