Dps will always be the deciding factor in group composition. Healing output and mitigation are necessities while dps output is the variable to maximize. Think of it as a problem where you want to maximize dps output while still meeting mitigation/healing requirements.

Even if the dmg on tanks are so high that you need to be in tank stance all the time, people will still try to optimize such that they have the maximal dps output while satisfying the mitigation requirements. The only way you can prevent dps from being the deciding factor in raid is to punish high dps groups with a counter dps check (wipe if total dps exceeds a certain threshold), which doesn't sound like a good idea to me.