Just to roll off of this, at 917 tenacity you get about 2.7% damage and mitigation increase. Conversely at just 868 Direct Hit, you get a 13.5% chance to do 25% more damage which works well with Crit since they changed it awhile back to make DH work off of stuff like Infuriate and Inner Release. Also 1861 Determination gives you an innate 10.8% increase. Can ignore crit in this equation for now since its around 2.1k for me, but that's what people mean by Tenacity not scaling all that well. You'd have to sacrifice a whole lot of Crit/DH/Det to start stacking up Tenacity...just to reach 8%.
Also as Rithy pointed out, we already have such a strong kit with things like Rampart built into tanks innately. Damage is king, it's just tricky on improving Tenacity since the only way it could be good is if it did enough damage to replace one of the big three. I've seen all the Mastery talks on the thread but I doubt the devs will do it simply because people are already lazy about materia in general let alone caring about deep systems. The guard rails are unfortunately remaining up with just; "Use this for more damage lol". Personally though, I'd love for materia and substats in general to feel less tacked on.