The issue isn't really the dps output(well it is for some obv, but it's not the main problem). They could buff overall tank dps to match the actual DPS jobs and it would still feel like shit to play a tank in comparison to actual DPS, because the role simply does not feel engaging to play anymore.
The gameplay for WAR/DRK has been significantly dumbed down and homogenized, PLD is more-or-less equal to SB version(they lost some but gained some) which isn't bad per se, but since it's always been the simplest tank it doesn't help much here and the shiny new GNB - announced as the "complex" tank supposedly - is really not that much different.
On the other hand, if you wanted to try to argue that it's fine for dps side of tanks to be easier, because they have other responsibilities to take care of - the core tank role gameplay has been simplified as well, even though it already wasn't all that complex in the past.
The aggro has been pretty much completely removed as a mechanic. Tank stances, while themselves were not super engaging, got replaced with free passive mitigation. This coupled with removal of ability to crit from bosses, made optimized cooldown usage way less impactfull. Mitigation cds were homogenized - there are still differences of general defensive strength between the tanks, but the need to change your approach to cd usage when you're switching jobs is mostly gone.
When it comes to encounter design, there's very little need to position bosses in the fights we've got. Making every single TB of the tier a double(seriously - every single TB in all savage fights), pretty much removed cd/swap planning which was prevalent in SB, outside of optimizing the invulns. Furthermore, the few somewhat interesting TBs we've got are timed in such a way that they can all be cheesed with said invulns. There weren't really any interesting tank-specific mechanics, like having to protect someone either.
There's hardly anything enjoyable left in the current tank jobs, both in terms of offense and defense, so might as well switch to DPS and at least enjoy more engaging damage rotations as well as far more job diversity.