The problem with adding forgery prevention features to paper money is that people could just forge the old version of the bill.
Old bills are eventually removed from circulation over time to the point where counterfeiting older forms of currency is ineffective because it is no longer accepted as a form of currency.
I think its worth noting that currency forgery is a business that operates on scale. Forging one $100 note is an enormous amount of work for not much payoff. The goal is to forge 1000s+ of $100 notes.
But at that point, you run in the question "Where do you spend it?" Unless your forgeries are exceptional, you can't just drop them off at the bank, precisely because banks will become suspicious if you show up with a duffle bag of 50 year old notes.
Fortunately, we have a number of methods for laundering currency - either by spending it where people don't look to hard or by mixing it with more modern notes so the 1000 notes you just printed don't stand out in a pile of 100,000s of such notes. Strip clubs, casinos (this was the original plot of Rush Hour), liquor stores, and other cash-heavy businesses do a great job of co-mingling fake and real notes. Black market trafficking is also a popular venue for disposing of dirty cash, as drug runners and smugglers are notoriously unsophisticated in detecting forgeries.
So, old bills can and do work as counterfeits in the right venues. And eventually, you'll see them vacuumed up and disposed of at the Federal Reserve right alongside the legit older currency. It's just a lot more work to do correctly.
I have known people who make counterfeit currencies and the most profitable and effective means is to make new bills, not old bills. This is because of what you mentioned: It is hard to not only do it, but to find a way to spend it. Old bills are more scrutinized than new bills by most business' so the effort is better spent forging modern currency.
The only place that will accept them is banks. And they have all the tools and training to spot forgeries
Yes, this is a major part of the removal from circulation I was talking about as money gets deposited into banks.
That's why old currency isn't valid after it's been changed though? That's why most countries have swapped to plastic notes at the same time as adding anti forgery things.
Maybe the US is weird then? Seems like old money is always valid:
https://www.uscurrency.gov/acceptance-and-use-older-design-federal-reserve-notes
I remember paper money. In my country there was a period of transition from paper to plastic money where both were valid forms. I had a similar thought. I made some old paper money on a normal printer, but I found an excellent material to use for the woven shiny ribbon. I gave the counterfeit notes to my friends at school for a laugh and they used it to buy things from the dairy and tuck shop (school cafeteria).
Actually worked surprisingly well, until the glue used to join the two sides failed. One note pulled apart and the tuck shop owner noticed while cashing up. The police came around to my house but we placated them with muffins. It turns out someone had tried flushing a bunch of notes down the school toilet and the police tried to retrieve the money from inside the plumbing system.
Fun times. I was about 11 years old. Had no idea of the seriousness of what I was doing. Cops found it pretty funny, but I remember they were still very concerned about the notes spreading into circulation.
The older the bill the more scrutiny it will receive so unless it's perfect someone will figure it out.