And Square disagrees based on how they designed the tanks in SHB. We can agree to disagree on this one, as its personal opinions.
You can assume what you want, I'm just speaking from the evidence that Square made tanking much easier overall as a response to how it ended up in SB. Sure, many players may not want the current system, but I'm willing to bet just from multiple friends I've chatted to and have enjoyed tanking since the beginning of SHB that many also enjoy this system. As to the 2nd point, jobs within role group have universally similar concepts. Making a tank that sucks at building aggro compared to the others would be the equivalent of making BRD have to stand closer to its enemy all the time to use weapon skills. It'd make for an alternative gamestyle, but compared to others in its category, there's a fundamental difference between it and the others.
Lets use an even better example. Remeber BRD back in HW when it had castbars? How it fundamentally changed an aspect of the ranged role and people complained ad naseum about it, especially when it was apparent MCH was built with castbars in mind, whereas they were obviously tacked on to BRD to create parity? yeah.
Rhetorical or not, the purpose of Square changing aggro was to make the role more popular to people adversed to tanking. They have literal terabytes of statistics of their playerbase and multiple language forums to funnel info from. When they decided to change aggro management, it's pretty obvious it was based on a statistical decision to benefit the majority, whether you personally enjoy the end product or not.
FFlogs from HW & SB severely disagrees with you. WAR was the absolute KING for basically two expansions straight. for the entirety of HW it had a permanent 10% damage down on the boss, making it basically mandatory for any serious to kill raid group in the early weeks. Then IR came in SB and especially in 4.2 when it got changed, WAR was just an unstoppable force in raids due to how absurdly it could generate aggro compared to the other two tanks without suffering dps loss, creating large rdps contributions with slashing, etc. and PLD in SB got insanely strong, and was in the vast majority of ultimate clearing groups. DRK got left in the dust in SB. It wasn't until SHB that WAR finally resigned from its throne.
Alexander Savage had 18k parses that involved a WAR compared to DRK/PLD's 10/9k.
Alphascape Savage had both WAR & PLD sitting around 16l-18k parses compared to DRK's 6k.
Let those numbers sink in. WAR was literally played in raids almost 3x as much as DRK in SB, and 2x as much in HW. So no...DRK was far from the 'most' played. Literally the exact opposite, Empirically.
You must have never been in a dungeon run that melted mobs ultra fast if you're getting rampart every pull. My bf gets rampart every other pack (or boss) because a good group can completely annihilate a pack in about 30-45s. Also, rampart is 90s vs the 120s (2min) you mentioned in your post for your theoretical OGCD. A small difference, but in a 12 min run of a dungeon, is quite a huge chunk of time relationally.
Also, considering how low threat gen for a tank is without their stance, I was giving you a simplistic reason why your tank OGCD idea was flawed. If there's no way to guarantee it'd be up for every pull, or worse, it can be unavailable for a length of time when you'd need it for the next pack, it would be a logistical nightmare to balance it's uptime in dungeons vs uptime in raids. Developers have to account for EVERY situation when designing something, no matter how mundane. As far as your DRK example, I wouldn't count that as flawed job problem, as it had an easy solution; save some MP. Or alternatively, turn off darkside between pulls, as Unleash was immensely low cost and you'd regen enough MP between pulls to use it. Whereas a single button, which when pushed, has a Cooldown till it can be pressed again, and would provide a tank with the means to do their job, could potentially NOT be up for a pack, is an actual flaw that would need to be fixed ASAP.
I have no doubts Square thought of many ways to solve the aggro problems to promote more tanks playing the role, but they most likely just simply took a look at player behavior (dps max aggro min) and decided to save themselves from their constant balancing headaches and made the system what it is now. Let tanks have fun with their rotations and playstyles without having to worry about aggro outside of initial securing it. Let them be the tanky dps they keep trying to be. Ultimately, the official forums are but a tiny fraction of the playerbase, only Square knows how popular the new system is, as they'll have the tank played % statistics across the entire playerbase to know if their system has worked or not, while we argue on mere conjecture.