Tanks that think 100% tank-stance up-time is something they should do are typically on the low-end of the skill spectrum. So, they'd be parsing well below your average Savage tank which was my reference point for those numbers.
No reasonable raider would harass a tank who is pumping out the tank-stance numbers I gave you. They might point out that you don't have to be that conservative, but I don't think they'd kick you from the group. But, a tank pumping out that type of DPS while staying purely in tank-stance would be an anomaly because at that level of skill, they'd probably be actively stance-dancing.
It's much more common that a tank who insists on staying in tank-stance will not fully understand the job and not have very optimized play. But, you wanted numbers based on a proper standard.
At the end of the day, you can do one of two things --
You can be genuine and merely want more tank archetypes.
or
You can understand how tanking in this game works.
You can't do both. If you understand how tanking in this game works and will work until SE discontinues their server support, you cannot make any intellectually honest and logical argument for the existence of a turtle tank. Any argument you make will be based on pure fantasy and personal delusion. Content will always be tuned so that any tank can clear it and as such, mitigation will always be catered to the lowest mitigation tank. Content will also always be tuned so that DPS is not completely trivial. You might make the argument that a turtle tank with less DPS would require less healing and enable higher healer DPS but that's just pure hypocrisy. You want to live out your fantasy of what a tank is but want healers to DPS instead of healing. You also cannot suggest that the DPS gap could be resolved by bringing another DPS instead of a second tank or healer without also suggesting that content design and job balance should be thrown in the trash can for the sake of your bias. That's the truth.