Of course there are no guarantees, but that is silly logic. IMO, by that logic, they shouldn't trade either because Scola/Odom/Martin could all suffer major injuries and never be the same level of player they are again.
Point is you have to give your team the best shot possible. If you trust Demps and you think he is a genius that can pull of such a sick trade, you can't turn around and say you don't trust that guy armed with numerous high draft picks to do well. Nothing is guaranteed, but you build through the draft and give yourself the best shot possible.
Drafting isn't all luck, the luck part usually is more about getting the luck to get high enough picks to draft good players (which NO would have). There is a reason why teams like the Spurs continue to draft stars no matter where they draft from. It's hard, but not impossible if you trust your guys.