Lets stop fighting SE and work with them. What are your ideas?

[Reynhart]

If tanks do too little dps then one of the two tanks in static groups would feel obligated to swap to a dps job for floors that don't require two tanks, otherwise they'll just be a burden to the group.

This already exists. The second tank bring no utility to justify its presence outside of forced dual-tank content, and all tanks do less DPS than actual DPS, even if the difference is "small". Increasing that difference wouldn't change anything.