Why does beelzebub worry that his eternal life may be of no use to him?

He is the right hand man of Satan and holds a high authority in Hell.

He had the breath of God all around him and was the second in command in heaven, his powers were not taken from him after the rebellion for wanting to dethrone God but however, he was driven from heaven and he was quick to join forces as a henchman of Satan to work against God's plans for mankind.