Ultimately the human mind can only store so much, and human beings are more than happy to settle in a routine. People already spend 20+ years doing the same thing every day, what's to say we couldn't spend an eternity doing something like that? Do we really only enjoy life because we see or feel new things? We would lose a sense of fear only after we experience something many times, and learn it does not cause us pain, or we learn to endure the pain and treat it as normal. Ultimately fear in humans is more attuned to culture and pain than it is to fear of death. Maybe some things we would fear more, such as being buried alive in concrete, thanks to immorality. The host cannot host an eternity of life, assuming these people continue to have kids. If we control the population and outlaw those who live forever to have kids, and say that those who have had kids cannot live forever there would be no issue.