Really, if Squall did leave the orphanage to find his sister and was never heard from again, well, Laguna wouldn't see him in his head giving him encouragement to take on Adel. Therefore, there would be continual war waged; Esthar versus Galbadia. Ellone would also be not saved by anyone, and in that case, Adel would have extracted power to add to herself. It'd be anarchy.

Even so, Galbadia Garden remains, so a protagonist may rise at a later stage and defeat Adel. Edea on the other hand... well, I don't know about her.