But there has been numerous things that have been gated behind achievements. 310% flying was only available with the said mount it came with if you remember that being in-game if anything, it made people farm the bloody instance to get those achievements and still was a while after until it was slapped onto to vendors. When flying was put into the game, well you had Outlands that was broken up so really it made sense as well as Northrend, especially in Storm Peaks being part of the questing you know? But after that with Cataclysm it was I feel more to do with reviving that old content and making it easier to traverse, giving people that luxury because it was old world and hadn't been used since well level 58+ and majority of the end game being away from I guess the mother continents of Kalimdor and Eastern Kingdoms if you get me. That's when it started to go a bit wrong and people got comfy.
I don'ty see it as fundamentally changing it but instead reverting back, in essence Vanilla only had flight paths and in Burning Crusade you levelled up through content and earned it. Same as Wrath, you levelled up through content and earned it too. Which is what is being done now... You go through content and earn it. Achievements is just an easy way for coding to reward you it really. Cataclysm was when it started to be given to people off the bat without much effort - pretty much the idea of welfare epics if you can see my train of thought.
Not having flying hasn't broke the game. Vanilla was that all the way through, the game coped in Warlords of Draenor, Legion and in fact every part of the game where you was levelling up and not using it. It must be a complete annoyance and actually worse though when you've spent a few years on developing content and have the user not even notice or interact with it because flying is like a quick escape from things.