Theres another exception to this, which its too late for FFXIV to adopt but woth pointing out: Games where the tanks are the selfish dps.
They've got the big numbers and need to use those big numbers to keep threat off the party, the "DPS" roles in those games then have supporting buffs/debuffs and maybe even some healing, and in the case of casters they could have superior area of effect damage, whilst bringing up similar ish damage to the tank, making their "rdps" as it were higher than the tanks. Never seen an mmo take this approach but it could work in a settings where you can only take one tank.