You really are into all this RP stuff ain't you?
Well the horde isn't that much better off as you may think.
Sylvanus's army wasn't huge like you make it out to be. It just had the advantage of being behind a giant wall; which is why everybody was so hesitant on the assault.
Dazar'alor is still in ruins, with most of it's inhabitants dead, both from inner conflict and the assault by Alliance back in 8.1. It also has no navy left.
Thundertotem and Suramar didn't exactly make it trough Legion intact. Why do you think they felt they needed to join a faction in the 1st place?