SE just listened to their playerbase when they were making all the changes to the enmity system.
Because of the nature of the game and importance of the DPS, tanks were asked to do DPS as well as healers were asked to. Players played the biggest damage dealing tanks because of it, and SE found out its not the enmity control what made people playing tanks, but the class performance itself and aestethics. Tanks were played in a way where they use as little enmity generators as possible and use the big DPS moves.
I dont know anyone who ever said "yes i am playing a tank because i like to control aggro", because it was not fun but annoyance, if DPS forgot to use enmity control tools a tank just had harder job.
Without tank stances, SE would have to really cut the design of most of the DPS jobs, since they were dealing so much damage and so many people playing them never used enmity control stuff they had, because of it it made tanks job stressful for some and more boring for others. Instead of doing great dps, tank were forced into tank stance if they wanted to hold the aggro, they were doing smaller numbers and people stopped playing them because of it, tank stance tanks were bad by a design.
You simply just cant have really enjoyable DPS jobs and a aggro system to manage for tanks. DPS numbers are really high, and if tank would not have too many aggro generators, it would be a total mess in the duties with tanks losing aggro and killing dps left and right like it was happening in WoW in old days.
With DPS doing so much damage tanks had to get aggro scaling on everything they use, and thats why we got tank stances in the first place, but what do we even need them for to be honest? If tank have to generate aggro on higher levels than DPS, and have to have enmity scaling on everything, then we could as well reduce enmity management to the small thing and release tanks designs, and this what actually happen in this expansion.
It was inevitable, tanks would have to get enmity scaling on everything anyway, otherwise it would restrain design of other classes, and this is why we have tanks in the current shape.