The transmog toggle is good but its far more important that we get a ui toggle. I cant stand the retail ui, and I dont see a particular reason why classic forever wouldnt atleast have an option to have the classic ui.
Blizzard is turning away a HUGE amount of players by not having a classic ui toggle. Just from a business perspective they should want this. I dont see any real negatives to having a classic ui toggle besides a small investment of time to make sure it works and works properly.