A little confusing since we always time travel to the future. If they were cryogenically frozen all that time, is that not the same result? If they had travelled near the speed of light (for some time)without warp tech would that not have dialated time and had the same result? If they had orbited a super massive object for some time would that not have also dialated time with the same effect? I could see the desire to restrict travel to the past, but it seems silly to me to simply restrict time travel to the future. What am I missing?