I think half of that is made up because in raids you'd aoe shit/trash down too.
I was a hunter, so I can't relate to any aggro issues (MD+FD). but since guilds could easily deal with that in Hyal and BT etc. I don't see the problem threat has.
And mobs rarely did anything other than hitting the tank with spells or autoattacks.
And saying that a lot of mobs don't hit hard when even the easiest instance - mecha-something-nazar had these big as ogre-like thingies with needles that would enrage(?) and nearly two hit the tank or something.. meh, I don't know.
The reason why CC was being used is because the tank would've died otherwise.
Anyway, as a DD, TBC heroics were easy. There was little to no engagement with any mechanics whatsoever. It's pretty straight forward when you can only deal with one or two mobs at a time anyway, due to limitations in either heal or tanking.
Legion M+ do at least require me to meet certain DPS values and boss design in general is obviously far more complex than it was 10 years ago. On top of that, some affixes actually need to be handled properly, all the time.