Yeah that's the issue. It would be have been better if it had a MUCH higher percentage but then it would be extremely powerful. Then again, it would allow some flexibility , although easier immortals could have happened.
Hyper armour is meh at best but it could have had more % to really be worth using in arena. I mean it's still probably used . Maybe at 18% or so could have been a bit better. Although in PvE, it would have a really high value. Ok maybe not that high but something.
Even in PvE , double mitigate has such low % . If it had been quite high, maybe more survival builds in floor climber could return as a way to add variety. Even though pure DMG still rules, making DMG Reduction way easier in campaign could be good second option if DMG reduction values are huge.
Dauntless helps for example , defiant , pathfinder and some others but if DMG reduction could get really high there, maybe more variety of floor climbing builds (although pure DMG will still Ultimately win since 100% DMG Reduction could be impossible).
I mean arena battles would certainly take a lot more time if DMG Reduction was that much easier than it already is (pretty easy right now).
But then again, high DMG builds in arena could greatly benefit for a chance to have higher DMG Reduction than possible . Although I think it's probably better if arena could be a little less tanky or a better way to end stalemates quicker.