The metal detector makes the airplane neither more nor less safe than the security theater porno scanner machine, and the precheck also doesn’t accomplish anything. The only reason most of the people need to be diverted through the porno scanner machine is that the federal government spent a few billion on them in a handout to some senator’s friends, and to scrap them now would make the tremendous waste of money obvious to everyone.
But at the same time, business travelers don’t want to go through the new machines, so we let them pay a nominal fee (easily amortized down to trivial if you fly a few dozen times per year) to go through the old metal detectors instead. Bonus: they now get to take a shortcut in the security line that they didn’t used to get. If someone without a real precheck manages to sneak through the metal detector line by counterfeiting some paper token, it isn’t a real security risk.