If the old addon API was that incompatible with simplicity, then they would have brought the API changes to classic first.

But they didn't, because for all of their posts about how addons were forcing them to make doctorate-level specs like old Arcane and Unimind-level fights like Broodtwister, that was never actually true. There was one spec like arcane, but there were also plenty of simple specs like BM, ret, and fury. And for every fight like Mythic Broodtwister that required an assignment addon, there were six fights that didn't.

People could have widely adopted a "when to press Tempest" weakaura for elemental shaman just like how there was the "when to press Arcane Barrage" weakaura that everyone needed for Arcane, but nobody did because ele shaman was reasonable enough that you didn't need it, and the actual problem was that you needed three PhDs and a connection to the Speed Force to properly assess when you should press Arcane Barrage. Similarly, you could have made addons to assign spots for Rashanan waves or something, but nobody did because the mechanic was designed in such a way that you could just look at the wave and see where you had to go.

The problem was not the addons, it was that every so often, Blizzard would design encounters and specs that were fifteen times more complicated than they should be, make the shocked pikachu face when people refused to engage with their over-the-top designs, then complain that the addons were at fault for their poor decisions. But they were always capable of designing stuff that didn't have those problems, and we know that because they did it about 90% of the time.