The CAIB asked for options for a hypothetical scenario. They were given options for a hypothetical scenario. It's unknown whether anyone would have had the guts to actually try to do it if the opportunity arose. Furthermore, sacrifice for the sake of rescue is one of the better aspects of humanity that is respected. Otherwise, all risky rescue missions should not happen.
A large part of the CAIB's asking will have been based upon the question of, "could NASA have done more to avoid the fatalities, after the issue had arisen?". If the conclusion of the hypothetical scenario was they could not have launched another craft in time, then it is obvious that, even had NASA known the extent of the damage of Columbia, they would have had no means to bring the crew back safely (though, of course, an Atlantis-based rescue mission wouldn't, per-se, be any safer, because Atlantis could easily fall foul of the same issue Columbia had).
