What do you guys think is the cause of severe lack of Tank users?

[Lyth]

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Tank stances aren't really a good example of a trade-off. The phrase 'trade-off' implies that you're giving up something of value. If you give up something which has no value, it's not a trade-off.

What happened with tank stances and accessories is better described as emergent play. The devs expected that we would turn on stance to tank, and turn it off when we were not. You mentioned ARR. In ARR, you would open in defiance regardless of whether you were MT or OT. Why? Because you could get the critical hit bonus as well as access to Inner Beast while you were in your Unchained window, which briefly gave you more damage than when you were out of stance. We used tank stance when the devs didn't expect us to, and we didn't use it when the devs expected us to. We used gear that we weren't supposed to. But instead of rolling with it, the devs kept implementing changes to our abilities and stat progression to force us to comply with their original vision. Tanks have probably had more revisions to their damage formula across the expansions than any other role.

By the by, most non-tank gear seems standardised such that your defense and magic defense stats sum to roughly the same amount, regardless of role. The one notable exception are dragoons (which is the obvious counter-example that you still haven't addressed), for the historical reason dating back to ARR that I mentioned in my previous post. But that has no bearing on their dps, because quite frankly, variations in defensive stats are not irrelevant to non-tank job's gameplay so long as they survive raid-wides, and thus no trade-off is involved.

If your claim was true, variations in defensive stats in non-tanks would correlate with variations in their damage output. They do not.

I remember that one time that the devs believed that they were creating a meaningful "trade-off" between defensive stats and offensive stats. What was it called again? Oh, right, Parry. Giving up something irrelevant for something of value is no trade-off at all.