The idea that Darnassus was the only or best way to transport Azerite to Eastern Kingdoms is nonsensical to begin with. But even if we accept that for the moment, while the Night Elves were busy rebuilding, the Horde could divert their military to entrenching in Silithus and defending the mining operation there. The Alliance can’t ship Azerite through Darnassus if they can’t get any of it.
Of course, that wouldn’t have worked either, because we now know that Azerite was going to start popping up all over the world anyway. But it would have been a sound alternate plan based on what they knew at the time.
I get that you think that, but the writers at Blizzard don’t necessarily agree with you. The point being, no matter how you slice it, Sylvanas’s plan was dumb at some stage. Either her original plan, on which the entire decision to go to war hinged, was bad because she had no practical plans to hold the city–or she had practical plans to hold it and therefore destroying it wasn’t necessary. Not even to break the Night Elves’ spirit, because as we are already seeing in the game, that isn’t working.
Of course, this is all assuming we don’t end up with a Big Reveal that she intended to destroy Teldrassil all along, and the reason she gave to Saurfang was fake.