What's the point of adding security features to new bill designs?
Bills don't last that long. The Federal Reserve estimates that $100 bills last about 15 years. Therefore, old bills are fairly rapidly rendered quite rare. Bills that look really odd (like the rare ones) are likely to warrant a second glance from the acceptor and later, if the counterfeiting is uncovered, much more likely to be traceable back to the forger. In The Great Paper Caper, Wells Tower tells the story of Frank Bourassa a counterfeiter who focused exclusively on modern \$20 bills because the enormous scrutiny on \$100 bills in general.