Well, OP is right. But the issue is, that its more of a symptom of the issues, moreso than the core issue.
As a frostmage, chances are, ill be running 4set 2set til KJ dies on mythic. Even with 940+ items on both my 2set spot with optimal stats, they arent upgrades over 905 NH tier. And even if they were, lets say equal, wich they arent, i still wouldnt use it, due to how the 2set just makes the whole rotation much smoother and more interactive. Atleast for frost-mages, theres no direct interaction to our procs from any stats other than haste, due to more casts. But that means any such thing that increases the chance of us NOT casting fucking shit frostbolt, will be insanely inflated in value to a point where it isnt even funny. The issue isnt that the 2 set is good. The issue is that stats are so god damn awful for some specs. Most specs with high scaling arent running 2set.. But those without any, will continue to do so.
Fix the scaling, add interaction between the spec and the stats you get. And youll not see any run around in shit ilevel 2set from previous tiers.