If a large number of new looking older bills start showing up it'll raise a lot of eyebrows so you want to get a better bill out so you can start getting the older easier to counterfeit bills out of circulation before counterfeiters figure out how to make them. It doesn't solve the problem for small transactions in whichever country since you can pass a couple older bills at a time. It does help for large cash transfers to other countries and for USD, Euro, Pound sterling, Yen, Franc, and Yuan because there's foreign reserves to think about. A large amount of US currency is held by foreign governments and banks as a foreign exchange reserve. If it was ever easy to counterfeit the latest bill a lot of the faith these foreign institutions hold in the bills legitimacy would fall apart.