Tank survivability is not meaningfully affected by Tenacity in the only environment where these considerations (or materia melding in general) matter, which is Savage and Ultimate content (and newly released EX primals for the first few days MAYBE). In these environments, what matters is whether you can save your healer from using a GCD on you, so they can do damage or group heal instead. No realistic amount of tenacity is going to change that right now, because all damage on the tank is either HUGE, massive tankbusters that require cooldowns (which are available at that point in time, as fights are obviously designed with a specific cooldown timing in mind), or it is part of a mechanic multiple people get hit by, and thus needs a group heal anyway.

Auto attacks and other fluff damage on tanks can easily be healed by fairy, OCDs or other tools, and Tenacity doesnt matter there.