I don't believe the expulsion is designed to be permanent. It also is meant to save their lives.
War is never a pretty thing. The devastation it brings often doesn't discriminate between combatant and civilian. I'm always in favor of a diplomatic solution first, but I don't see one here. Because Hamas uses civilians as shields for its weapons and fighters, there are going to be civilian casualties. Israel should do all it can to minimize civilian deaths. But if Hamas isn't taken out, the number of civilian deaths will never stop.
Having said all that, once Israel controls Gaza again, it is incumbent upon them to ensure that no terrorist organization is allowed back in. Otherwise, rinse and repeat.