If checking for counterfeit bills becomes harder, trust in the currency is degraded. Once that happens, vendors will either incentive digital payments (which are definitionally tracked), switch to a tracked currency, or barter more. Trust in bills is super easy to take for granted.
US bills use both, and the invisible watermarks have proven their value. Imperceptible imperfections intentionally (yay, illiteration) placed on 100 dollar bills have managed to catch advanced North Korean forgery attempts. Same goes for magnetic inks and all kinds of visible and invisible watermarks on US currency.
