New bills seem like a wasted effort:
• it doesn't matter how many times they redesign the bill or install security features. The counterfeiters are just one step behind and will catch up in very short order.
• more to the point, paper currency does not "expire" or lose its face value even if the bill is very old. If I stashed brick of c-notes in my safe for 50-100 years and my kids (or grandkids) discover it later the paper money is still worth its face value. Approximately one half of all U.S. paper currency in circulation is circulating outside the United States. Some of that money is counterfit and some of it is printed by bad guys who have copies or near perfect copies of the "plates" used to print the money. Even when money is redesigned the old plates are still good for printing "new" money because the design of the plates effectively never expire.
The bureau of printing and engraving used to print $1000 bills and even $10,000 bills. If you have one or a stack of those bills they are still worth their face value even though those bills have been out of circulation for decades.
