I think tanking is more fun now then it ever has been, active mitigation works in differentiating a good tank from a bad tank the same way threat used to. WoD might push some tanks away who loved doing high dps by cheesing vengeance but those were not real tanks anyway imo.
I don't enjoy tanking tanking for people in random groups so I definitely understand why there might seem to be less tanks, I think its more that the competent tanks are more likely to be in a decent guild raiding progression content or avoiding lfr by doing flex.