Or the keys lost/forgotten, like Satoshi's approx 1M Bitcoin (possibly/probably). But this is a feature not a bug - according to Satoshi himself "Lost coins only make everyone else's coins worth slightly more. Think of it as a donation to everyone." (https://satoshi.nakamotoinstitute.org/posts/bitcointalk/thre...)
https://www.ecb.europa.eu/press/pr/date/2025/html/ecb.pr2502... https://www.bankofengland.co.uk/statistics/banknote https://www.cranecurrency.com/news-insights/85-decline-in-us...
Even making forgeries that can fool normal automated tests, like those done in an ATM, is incredibly hard. It's certainly possible, just way too expensive.
1) We have a legal system which (not perfectly) can reverse or fix issues which arise, with crypto money, it's just gone.
2) Someone who can produce fake colourful paper will typically only be able to produce a fairly small amount and not destabilize the whole currency. In the rare occurrences where someone found a significant hack in a crypto-currency, it's just done.