Yes. That would be a problem because then it renders 60% of the jobs in the game moot.
But in that fantasy land, you'll probably have arbitrary checks in place to keep it from being 2 tanks and 6 healers - Much like what has been suggested in this thread.
So while each healer is outputting 45,000, you still only take two, and those two healers outputting 45,000 are going to be expected to output 45,000. And the boss HP will be tuned to expect 90,000 from the healers, 18,000 from the tanks, and 60,000 for the DPS, and no amount of healer optimization will cover the 6-12,000 gap from underperforming DPS.
So in the grand scheme of things, what really changed?
And that's the problem.
-Contribution- from the tanks is fine. It's literally fine. Again, I urge any tank who thinks otherwise to go in, and slash off 20-30% of what you usually do. It might not mean much in your static, but do it in a PF group. A learning group. A clear group.
See just how much weight your contribution actually holds, let alone during the first few weeks when the content isn't overgeared to hell and bored 99ers aren't joining rando groups that settle for less than 2chest.