Feelings on FF14's Trinity System?

[Absurdity]

Why is it a "punishment" though? If it's factored in as an expectation of the role, how is that any different than the difference in DPS between roles, or even sub roles of DPS?

Yes, because this isn't a difference between roles or even sub roles, it's the exact same role but you do less damage by actually performing your role. Like I said, that's just old tank stance but with no option to turn it off.

Also, you do have an entire rest of a kit to potentially play around with. While one action might offer less damage while main tanking, another could offer more. You could have multiple tools that shift in value depending on what position you're at as a Tank, and trying to reserve resources for when you're one or the other could become a new aspect of skill expression.

Your initial proposal included none of that. You just talked about an attack doing either more enmity or more damage depending on whether or not you're main or off tank.