It is powerful heals that allow you to ignore non-lethal mechanics, because the damage you took has no consequence. The healer won't run risk of going OOM because he has to heal the damage you took and he won't risk having to let the tank die because he's busy fixing your error either. He'll dip out of cleric stance, hit Essential Dignity/Lustrate/Tetra, maybe throw a HoT and then go back into cleric stance. The error is fixed in under 5 seconds with one or two casts at negligible mana cost. This trivializes any mechanic that isn't completely unforgiving and why all "hard" encounters rely on lots of insta-kill and wipe mechanics.
As for mass pulls - that one is a little bit more complicated because it's indeed a matter of incoming healing vs incoming damage, as the stream is continuous. So you "can" theoretically work with increasing the damage, reducing the mitigation or reducing the incoming healing.
Reducing the mitigation might be warranted to some degree, but it makes tanks feel less tanky and the mitigation tanks contributes far less to the effective HP than healing. If you heal a tank with 15000 HP that mitigates 50% of all incoming damage for 4000 HP, you have contributed 8000 effective HP to his health pool. In order to achieve the same effect, he would need to get 4000 HP via gear or mitigate 60% of all damage - except then the heal would be worth around 10k effective HP. And that's just a single heal here, not multiple. That's why 3-man runs without a tank can usually continue with little issues, whereas 3-man runs without a healer result in people dying, wipes and standstill.
The issue with upping the damage instead of nerfing the healing is that it only makes things more unforgiving - In the extreme case of powerful healers and powerful damage, a healer could heal to full every second and mobs could get you to 1% every second. Now that's stress-free gaming, right? We are already at the point where healers can crit you for over half your health in a single, spam-able spell. How hard do you want to make mobs hit to match that? At this point, the issue is healer power, not the incoming damage. It's not one bit surprising that despite their low mitigation, healers rank highest in PotD. They are also the only role that has a skill flat out disabled in PvP for balance sake. How much more obvious do things have to get? >_>