The problem is that Nuclear Fusion is always 30 years away. It was 30 years away in the 1970s and it's still 30 years away today. Can't rely on a solution that would probably not be available in time.
The only good thing about time travel is that once we discover it we always had known it.
You can't predict the exact rate of technological progress, that doesn't mean it doesn't exist. We were further away from the goal 30 years ago than today.