I think the GP is absolutely right, Blizzard went to far with their streamlining, and every other MMO copied them.
For me the key difference between modern and classic wow is the design approach to challenge. In classic you go at things expecting to fail, in modern you go at things expecting to win. The difference this makes is that classic makes you happy while modern makes you sad when your expectations aren’t met.
Part of what makes that possible is the player freedom. You can make different builds, you can approach content differently and you can overcome challenges your own way. It’s like Blizzard, and I think modern game design in general, forgot that being challenged and figuring things out is fun.
I think I may have forgotten too, because I’ve been surprised at how well classic holds up in 2019.