Garrosh wanting Ashenvale wasn’t for the entirety of Ashenvale, he just wanted to harvest some of the forest for the benefit of his people. But when Darnassus refused, he decided to invade as a ‘no choice’ option.
Garrosh’s decisions and Sylvanas’ are totally different. Neither had anything to do with the faction but with their own agendas at the time.
So, no, Teldrassil was fated to burn in both story and dev.