In a world where tanks do the same damage as DPS you simply balance it by exemplifying the idea of rDPS.

DPS would bring the ability to create openings or weaknesses by either striking at certain boss parts causing the boss to take X% more damage or even just bringing more powerful buffs.

i.e
Tank does 100
DPS does 100 but brings party wides buffs or mechanics to enable more damage. (Let's say 100 more RDPS each)

A standard team of 2 tanks and 4 dps would do (100*2) + (200*4) = 1000 rdps

As opposed to a team of 6 tanks which would just be 100*6 = 600 rdps.

40% raid less DPS for 4 more unnecessarily durable players and far more limited LB generation? (More if we count role bonuses)
Yeah this would only be viable for severely outdated content. *cough* BLUE MAGE REASONING *cough*

This gives DPS a role unique mechanic of hugely boosting RDPS and then you just balance content around that so that non-standard comps are unviable. This then allows each tank to have equal agency to each DPS player in contributing to winning the fight by defeating the boss. All that happens is that now DPS are responsible for new forced DPS mechanics or RDPS much like how tanks are responsible for forced tank mechanics now.