If parry scaled too well, we'd faced the WoW tank scenario where stat bloat led to near impervious tanks under certain conditions. In order for content to not be trivial, stats must stay roughly static. Tanks in i90 vs i110 gain more hp but overall mitigation increases very little. This means that only incoming damage must be scaled in order for SE to keep content challenging. If you had no problems without parry in 1-5, nothing should change for 6-9. If you stacked parry and it worked, nothing should change either.