I expected Anakin to die in the lava. I expected Sidious to bring him back to life using the Dark Side, the catch being that Sidious has to consciously channel the Dark Side for Vader to remain alive. That is Sidious' power over Vader ... the true "Power of the Dark Side" ... the reason Vader "must obey".

Plus, if Anakin really dies, Obi Wan would not have to make the decision to walk away from his dying friend. And he becomes less of a liar in ANH when he tells Luke that Vader murdered his father.