You have it backwards. The goal was to make the dragon characters of WoW playable (Alexstraza, Kalecgos, Wrathion, etc.). Since you can’t play as a massive four legged dragon, they created bipedal, player-sized dragons, and created lore around those new models. Beyond size and number of legs they walk on, there’s little different between a dracthyr and a dragon. Evokers even have the abilities of some of Warcrafts dragon characters.
In short, you don’t get to the Dracthyr without decades worth of dragon lore to base them upon.
Not really, they just used HotS instead. Evokers have several abilities from Alexstraza, Chromie, and Deathwing HotS.