Adjusting levels of DPS output is a step towards changing the game to more mechanics-based content vs heavy DPS checks all the time. I want more stuff like T5, where the DPS check didn't dominate the mood of the encounter. Or Titan EX, probably my favorite primal fight to date.
I...don't understand the connection you are trying to make here. They already claim fights are designed WITHOUT the upper limit of tank's dps taken into account. Outside of two fights that they claim were overtuned, DPS checks are never the focus to begin with. It's always been mechanics already. What fights are just an HP sink? Even AS3/AS4, the "overtuned" places, had some very important mechanics.
Let's say 1k vs 1.5k+, rather than 1k vs 1.2-1.3k-1.5k. Could be an upward adjustment for the lower tier DPS classes or adjustment for tanks. More importantly, the point is if they nerf tank DPS there has to be a compensation for it to make tanks worth playing. So the solution would be to make dungeons more mechanically inclined than just raids. Yeah, they said it's supposed to be casual content, but at this point in the patch cycle a majority of the player base has at least enough base experience to start forcing more attentive play. I really enjoyed Steps of Faith, for example, as a step out of the box.
There's gonna be a lot of early frustration with a change like that, but in the long term it would make the game more enjoyable.
Again, tank dps (in regards to content design) is already assumed to be way lower than what is actually possible. They would not change anything as "compensation" because to them there would be nothing seriously lost.
You are just making a crazy assumption/connection here that fights are designed the way they are because tanks deal decent damage. They have nothing to do with each other. Just making fights more mechanically minded would not alter anything either, it would require a fundamental change to both the entire tank skillset and the entire basis of how the entirety of incoming boss damage is done. Like you mention T5 and Titan EX, both of those have the exact same design as we got now. Steady damage with specific burst you pop a cooldown for and that's it. That entire design is why the tank DPS meta exists, damage is so predictable and spaced so that you can safely focus on maximizing dps while only "tanking" for specific hits.
Suddenly lowering tank damage to be ~35% of dps doesn't suddenly change anything (why is that arbitrary number chosen?). Fights aren't designed with their dps in mind, and the meta would remain the exact same, just numbers would be lower. Nothing of how things actually play would change.
As for more challenging dungeons, I'd love that, but it's never happening unless the hard versions are their own separate thing. You vastly overestimate the skill of the playerbase, or even their desire for more challenging content. Steps of Faith was widely negatively received, and got nerfed into the ground fast because of it. AK normal was nerfed repeatedly to the point of no longer even being fun because of complaints. 4 mans are brain dead because the player base widely called for them to be brain dead.
But still we go back to what the hell does this have to do with tank damage. I seriously cannot follow your train of thought there.
Oh my god, DRK 50 quest is hella cool.
30-50 DRK line is best job quest line in the game. I'd probably argue one of the best period.