SM doesn't drop their idols, successful or not. This agency never dropped someone because they are not successful.
Each exo member is far from not being successful. That being said SM doesn't really do anything for exo neither exo members need SM. All opportunities they are getting now is because they have enough recognition, talent, popularity and influence. Because exo members are in charge of their own mysic they don't need SM producers and SM mostly hires producers from all around the world anyways (plus every producer dream is to work for someone like exo).
I don't see anyone leaving EXO, but for SM we will have to see.