You missed something in my previous posts. And that is this; the STR build initially had the better AC.
As a result it was the STR build that was getting hit only 25% and the DEX build that was getting hit 50% (to use your hypothetical situation) in the old system. That began to change as both characters reached L20 (still pre U14) as the DEX build finally overcame the STR build in AC. When U14 hit the DEX build had ~4 AC more than the STR build. And, until U14 went live the STR build was still preferred by me for running all existing content.
With U14 running through to Eveningstar on epic normal was no challenge to the DEX monk. It was challenging on the STR monk. Now, you could be right that it is something more than just the AC change. But, that is what it seems like to me.