So, cause I kinda nerd out on this stuff, I read the 2020 specs as well and gleaned that the 450 pickup is a wide track 350 with bigger brakes, 19.5s, deeper gearing option and a heavier front axle rating on ford paper with the same rear axle rating on ford Specs.
But the M300 in the 450 has larger shafts, more splines and "may" be in the 16klb category vs the lighter duty M300 in the F350s which are 11,500lb rated.
You don't see the "big" S110 axle in the 450 p/u, becasue the newer M 300 axles are heavier duty already than the old 1 ton axles. Suspect the same for the front axle but didn't read up on it.
Ford basically built a wolf (class 4) in sheeps (class 3) clothing.
Rest of the drivetrain, rear springs and frame are the same as 350 which is much improved over the old super dutys.
The lesser payload of the 450 is only about the difference in the additional weight of the wheels maybe?
So the question, why not offer a billy bad@ss 450 pickup with an even heavier rear axle? Answer, because how much more than 15-16k could you possibly need in a pickup...
It seems to fil a niche and pulls some sales based partly on "marketing" and partly on the function of a, realistically, upgraded 350.
Hot shotters, big RV pullers/haulers and guys that gotta have the baddest cowboy Cadillac, this is your truck. Stays in the class 3 designation which saves some hassles with reg, insurance and driver requirements (not really on the driver req) and provides the most capability out of a "1 ton" on the market.
A big "pickup" like this is not targeted to most of the "commercial" market, as a pickup bed is just an expensive liability with less usefulness than a flatbed, waiting to get smashed, in most commercially used 1.5-2.5 ton trucks. And a straight frame is more easily customizable for the commercial market.
I say Ford did a very good job "creating" and "filling" a void in the market that costs them virtually zero additional tooling or expense.