I really don't see how you can say that. Arth played Sylv, Valla, KT, Jaina, Sonya, and what in TS? I've never seen Arth pull out Zeratul or Thrall to my knowledge, though I may be wrong on Thrall.
Fan was the melee assassin for C9 and ran the gamut of whatever heroes were viable, then became the flex and played all the range while iDream went melee plus he has the best Abathur in NA. The hero pools are not even close IMO. It's hard to argue with the fact that won worlds with Fan swapping roles all around and are only 1-1 in actual tournaments that matter with Arthelon in the fold from my POV. At best it's a sidegrade, which is what I said at the time. I would assume Fan wanted to centralize shotcalling and Dunk is all about free-flowing and that was the main reason for the change, not an actual increase in skill.