No one has ever said that you can't clear Savage or Ex with tenacity melds in the history of ever. Problem with tenacity is though does not give a big enough boost to defence and offence combined to make it worthwhile for most people.
Some good reads about tenacity can be found in a couple reddit posts from this year:
https://www.reddit.com/r/ffxiv/comme...e_breakpoints/
https://www.reddit.com/r/ffxiv/comme...to_be_avoided/ (the comments sections)
And a quote with a couple good questions:
I would go for tenacity myself if I found it worth it but instead I meld DH and Crit to help meet DPS checks since a lot of random DPS can't play their jobs at even a decent level as indicated by the boss hitting enrage. Most fights don't have difficult DPS checks to make if all the DPS played at a decent level but most random parties will have one (maybe two if lucky) superb DPS while the other DPS will have far below acceptable DPS levels. Been a fair few fight where we barely beat the boss by the skin of our teeth, a few times actually party wiping as we killed the boss but still counted.
- Will it reduce incoming damage to the point where healers gain enough GCDs to offset my loss in personal DPS?
- Will it reduce damage enough to stop me from dying?