Would you mind telling me more on how you see this 'owning a niche' playing out?
Just off the top of my head - there are a finite number of niches (although probably expanding with number of people, they are at least asymptotic after mass adoption, approaching zero new possible niches). So early movers get the niches, maybe they can sell them if its profitable and the responsibility becomes too much? Or how will 'lazy' owners get booted? When you die in from a whitewater rafting mishap in the year 2093 who will run the speculative fiction niche?