when you're in the highest weight class you don't have to add the P4P. if you're the best you'd just be the best in the world.
well except that the skillset of heavyweights in MMA is very limited
If that theory were even close to being true, I guess you would consider Butterbean or Hong Man Choi the best P4P world
Just because you weigh more, doesn't make you a better fighter, or have the better fighters.
P4P in MMA is just too damn hard to call, there are way too many factors to put into place to decide an overall #1, #2, #3 etc. etc. One night, one fighters BJJ and takedowns might be on and could easily overwelm a striker, while the next fight that striker's TDD and striking skills could KTFO out of the grappler.
That's why in boxing it is much easier to declare a P4P King, simply because boxing doesn't have a vast amount of fightings styles to account for.