You're missing the point.

We can be set to do x% less damage than dps forever and that would be okay. If dps are always 30 % higher then a tank, then that works.

The problem we face here is that dps will start at 30% higher. Then will be 40%. Then 50% higher. Then 60% higher. Eventually and with enough time and gear upgrades dps could be 2x or 3x more dps than a tank and this is a real problem.

Later down the line they will need to constantly buff our threat output to hold aggro against such higher numbers.