There's a funny thing about gauging this sort of health-sink survival thinking. It has built in assumptions that don't often get rehashed or reminded to people because the people who have been doing this for a long time take it for granted.
Often we assume the healers' input is simply as much as is needed and either they can go forever, or they designed their gear to go as long as is needed. I wrote a whole thread about this that is far more sticky-worthy than this (as the specifics will change in this particular relationship endlessly). You simply need to recognize the needs of each stat:
1.) Your total health needs to be high enough to take the hits you're getting (see: mitigation). You must survive 1 hit, you really should survive 2 hits, and if you can survive 3 hits that's good but becoming less frequent, all without heals.
2.) Your mitigation needs to be high enough that your total health is sufficient for the above standards. It is simpler to see things in terms of Effective Health in that respect, but then you never actually see the boss's unmitigated damage so it is less helpful in that respect.
3.) Avoidance allows you to break up the incoming stream of damage to create longer spaces between hits, or a less consistent stream of damage. This allows for catch-up if healers fall behind or are distracted. It also is independently responsible for a lot of damage avoided.
Overall, mitigation and avoidance are terribly important for the amount of healing you require. If you could have 200,000 health but no mitigation or avoidance, you would not be more pleasant to heal, you'd be a mana-sponge. Just because that makes it predictable does not mean it makes it more pleasant for the healers.
So from a healer standpoint, mitigation is what determines how hard they have to work, how much mana they have to drop, and how much attention you need.
If I had to guess about what we'll see in the Clysm, I'd say you can expect our avoidance to drop substantially, and for bosses to become less a matter of stepping in front of the 18-wheeler and doing everything you can to weather massive blows. The focus will move away from hardcore tank survival and towards the whole group having to navigate the encounter more equally. But that's just me guessing.