While I'm definitely of the opinion that tank DPS can be balanced against tank DPS while maintaining fairness and job identity, it's not an inherently wrong idea that you could have one tank that excels in party defence and pays for it with lower DPS capability while still giving all tanks an important and valuable role in the game's meta. The gap in party defense just needs to be really stark (imagine if SiO were taken away from WAR and given to PLD).

That's basically the situation with healers, where WHM/AST is best for progression on easy/recoverable content, WHM/SCH the go-to for very difficult content, and AST/SCH is the speedrun meta.

Not what I'd like for tanks but it's very viable.