Heavyweight is not a different game - its where you go when you want to fight older, lesser skilled, less athletic fighters. Thats why DC, the very same guy, can absolutely WALK THROUGH heavyweight, including summarily knocking out the champ. It has the oldest average age of any division. Are you familiar with any athletic sport in the world where having the oldest competitors is a good thing (read: its not)
You're mixing up examples where DC artificially reduced his weight to fit an arbitrary weight limit, compared to when he fought at heavyweight with others willing to fight at their natural weight.
I'm personally in favour of a free weight division, as it would really highlight who is only successful because they're good at weight cutting. Of course DC will get worse at weight cutting the older he gets - and we already know he had health issues when he was younger with weight cutting!