Complex sentence: At the end of the story, Forevers realize that they cannot save humankind, so they leave them freewill., but there is one man (Futureman/Colonist) alive on Mars, so Forevers give him the Dream Sequencer.
Split into simpler sentences: At the end of the story, Forevers realize that they cannot save humankind, so they leave them to their own devices. Though the human race may be lost, there is one man (Futureman/Colonist) left alive on Mars, so the Forevers give him the Dream Sequencer.