keeping everyone 21 feet away impossible

http://limatunesrangediary.blogspot.com/2012/02/twenty-one-foot-rule-be-hanged.html

That being said, whenever I see self defense situations discussed on forums, facebook pages, Youtube and the like I am overwhelmed by how many people spout phrases along the lines of, "If you let someone get that close to you, you have already failed," or, "I would never let someone that close to me." Then someone, somewhere, will bring up the Tueller Drill (or, in lay terms, the 21-foot rule) and say something like, "If a bad guy gets within 21 feet of you you're dead!"