I dunno. In general I agree with what has been said, but I can see certain caveats.
I keep a modest cache of $20s and $100s for emergencies. In the past 10 years, maybe half of them have become new style as they were used for unplanned pizza/chinese food deliveries, an occassional impromptu gift or short-term loan to family or friends, etc.
‘Cash emergencies’ are increasingly rare now. Grocery stores and almost everyone else will take credit/debit cards, 24-hour services are more common, and my finances have gotten stabler as my career advanced - so the turnover rate is pretty low. I think the last time I touched the stash was on Christmas, for a delivery from a certain restaurant that wouldn’t take a card for deliveries.
I rarely touch my stash of $100s at all. Though my grocery trips often top $100, they are still a fairly uncommon denomination for most other casual cash transactions - or maybe that’s just my impression.
About 10 years ago, several of my friends used to enjoy roadtrips to a then-new casino in a neighboring state, and I’ve had a pile of $100s since then - my initial 'weekend stake, multiplied over many weekends (I only enjoy gambling as a mathematical challenge, so I have no problem walking away when I’m ahead; it’s more fun to watch your friends play, with winnings in your pockets, than to play ‘catch-up’ an hour later when you’re behind) Gambling is a surprisingly large, cash-intensive, sector of the entertainment industry now, so I bet there are lots of people with stashes like mine. back then, it was just about the only time I used $100s.
Cash in active circulation might be 90% or more of the bills issued, but over time, a sizeable percentage of bills get stuck in backwater eddies and sumps in the cash flow - like my stash. These are less sizeable than they once were, due to the increase and ubiquity of financial services (ATM and credit cards) and you might rarely see them - but let times get tough [which is precisely when people might be tempted to do some amateur counterfeiting] and those emergency stashes will start flooding back, at least temporarily. After a natural disaster or terrorist attack (like the WTC), the banking channels may be tied up or overloaded, and the immediate need for emergency purchases will flood the stores with old bills and quickly kill the suspicions of the now-numb (by the disaster and the flood of old bills) store clerks.
(It occurs to me that a canny crook might keep a stock of counterfeit ;old bills’ for just such n opportunity. Not all crooks are dumb)