I actually find the inclusion of this as a "most likely" scenario mildly offensive, since Bostrom explicitly says he finds it seriously improbable:

"In any case, the time-scale for human natural genetic evolution seems much too grand for such developments to have any significant effect before other developments will have made the issue moot."

There's something... unpalatable about intellectuals bemoaning the "lesser folk" from breeding the species into oblivion, particularly when it seems to contradict both evidence and theory.