It's does not destroy estrogen nor does it destroy ALL of the estrogen. It blocks the aromatose enzyme from doing it's job, but the effect in an ideal dose will be close to 90%. So taking a proper dose is important. Too much is not good as is too little.
The problem with nolvadex is that you still have all the estrgen circulating in your bloodsteam, but it's being selectively blocked. Now keep in mind that a lot of the estrogen produced and aromatised happens in the fat cells themselves, this is where blocking this action with a drig like Arimidex will make it easier to "dry" fat cells and make fat loss easier and more pernmanent.