I honestly would expect the game's longer-term design to end up being everyone becomes more of a DPS with bonus features than anything; their lack of a PTR and general inability to get a good read on what players will actually do with the tools they give us means they're going to have to get more and more conservative and narrow with time.
Would increasing the damage mitigation given my Tenacity help curb some of the issues tanks are experiencing?
So the fundamental problem is that mitigation and more HP alone don't do anything; the dividing line between "useful mitigation" and "pointless mitigation" is, essentially, if it saves the healer a cast or turns something that would be a one-shot into not a one-shot.
If a boss attack does 9,000 damage to a tank, and they stack 1,000 secondary points of Tenacity, they get a slightly-below-5% boost to their damage and a slightly-below-5% reduction in the damage they take (as far as I've seen it doesn't boost actual healing received aside from self-heals). So instead of taking 9,000 damage from that hit, they take 8,550. Yes, the tank took less, but I'm probably going to heal both situations the exact same way. If I handle both the exact same way, my preference would be for the tank to take those 1,000 points and figure out how to turn them into Direct Hit to get a 6.67% damage boost instead of 5%-ish. It's not huge, but it will directly contribute to the thing dying faster.
In theory there's a crossover point, where tanks could be passively mitigating enough with Tenacity to save me actual heal casts and thus be benefitting everyone, but it'd have to be a pretty substantial boost with the way HoTs and such work in the game.
I'd honestly be totally fine with them just making Tenacity a god-tier secondary since they've made it impossible to meld onto non-tank gear. Overweight the hell out of it, give it strong damage boosting and mitigation properties, and let it cover for the missing STR on accessories. It would be a borderline broken most-important-secondary-ever for tanks, and they could just auto put it on accessories. But it should still heavily impact tank outgoing damage, because for the most part a consistent boost to outgoing damage is going to outweigh saving a healer two or three GCDs in a 10 minute fight.