Well I think calling someone's build "weak" is bad form. The best build for a person is a build they enjoy playing and pushing solo skulls isn't the success criteria for every player (very few rank this high in fact).
I also think you are making assumptions about what he runs R1 and R5. I personally run legendary Sharn and Ravenloft on the exact same skull every life, but there are some specific quests I run at lower skulls or skip each life because of the specific quest mechanics or I don't enjoy it as much as I like the other quests. An example for me is Best Laid Plans, Slavers III, TOEE part II. The latter 2 are not tier 2 but I find them harder than most tier 2 quests.
What is your basis for this conclusion? I am not seeing the impact of these changes being so significant.