Cant we all agree that Sylvannas burned the tree to get the elves out of kalimdor and limit their invested interest in the *west*? If she could burn the exodar id light the match myself but im not sure about how flammable crystal spaceships are...
All of it was an elaborate ruse to trick the boy king and his dog into attacking lordaeron where she had rigged the whole place to blow years ago, thus decimating the alliance forces at the cost of a smaller capital city. Its like saying "Oh I'll take the big house on the West side of the street, you can live in the one I've been living in on the East side and btw I virus bombed the whole attic. See you next war..."
I think its tactical genius.