So the Icebound Fortitude is just as good for the fresh 100 tank in quest greens as it is for the full BiS Mythic geared tank. I don't see why that is a problem. Maybe I'm dense. So please, spell it out for me why it's bad that a spell with very minor uptime that is meant for emergencies or strategic mitigation of massive incoming damage (like a dragon breathing) has %-based mitigation that is just as effective for the fresh 100 quest geared tank as it is for the Mythic BiS tank.
It's a problem because healers don't scale that way. Healers gain higher output and resource regen as the expansion progresses, which changes the actual way their class functions, and drastically alters the level of damage they can heal through at a rate much higher than the rate tank effective health and avoidance stats inflate.
The result is that bosses end up tuned increasingly towards the "burst to death" point. Under the current healing system, that's largely been okay, because we don't have triage healing. Your healers are expected to keep your tank at full, because a string of unavoided hits WILL bring him down. And bosses are tuned to their healing capability, so things feel the same for healers.
But remember, this is okay SPECIFICALLY because we're playing a game where healers keep people, most notably tanks, topped off. Because they get burst down quickly otherwise.
When we move to a triage system, that system stops being okay completely. You can't tune to healer output in the same way, because tank healing is no longer a game of overhealing. Now it's a game of mana management versus survivability.
Under that system, damage levels that scale with healers, not with tanks, creates a system that is functionally identical to early Cataclysm healing
. Healers are now forced to use an overhealing model, to keep burst from bringing tanks down, in a model where they were supposed to be relying on playing the health bar game. And it just doesn't work.
Let me put it this way:
Joe is a tank, and he has 100 effective health and 25% avoidance. The boss hits Joe for 30 damage. It takes Joe 4 hits to go down, though there's a good chance it would take 5.
Sally, the healer, has her healing balanced around that 30 damage number. For simplicity's sake, let's just say she heals 30 hps.
Not lets jump forward a patch.
Now, Joe the tank has 120 effective health and 30% avoidance. But Sally now has 45 hps as a combined result of her increased mana regen and output stats. So now the boss needs 45 dps to deal with it. Now, Joe can only survive 3 hits, and his chances to extend that to 4 aren't as good as his chance to extend 4 to 5 were.
Joe's cooldowns and such are just as effective as a percentage of his effective health, but they are now LESS effective as a relation of his effective health to boss damage.
This has been a pretty consistent "problem" with tank scaling forever. Except that it wasn't a problem, really, because bosses were being balanced around healers, not around tanks, and the shrinking window of tank survivability was less of an issue because of dedicated tank healers. Since healing was so heavily based around overhealing in the first place, tank's time-to-die was irrelevant. It doesn't matter if the tank goes down in 4 unhealed hits vs 5 if you're keeping him overhealed anyway.
But when you eliminate that overhealing factor, when you make healer mana an issue and expect tanks to not be sitting at full health, shrinking time to die suddenly matters in a way it never really did under an overhealing model. That HEAVILY changes the game.
If a healer is expected to only keep a tank at 75% health, then it's a big problem. Because as the expansion progresses, boss hits take a bigger and bigger percentage
of tank health away (not just larger numbers proportional to tank health), which means that healers are forced out of triage healing and back to the burst model...
...Except that they're completely different. Which is how you end up with Cata's healing mess.