If you want them to change the encounter design, why not just make the boss do more damage so that having vitality and defensive stats actually matter? Tanks are supposed to be mitigating damage to the best of their abilities. It would also make healers work more. You get to feel more like a tank without doing extra damage.
When I ask people why they aren't asking for more boss damage, they will reply that it doesn't change the way they play. Seeing bigger numbers will not change the way tanks play in raids.

Also if tanks did 75% of dps numbers people would just go with full tanks instead of dps and simply ignore mechanics.