an interesting article , the same thing was discussed in the topics about how to store your coins safely
the technology might not age well , but I suspect USB sticks won't be out of date in 20 years
I'd be more worried if bitcoin itself stayed relevant , anything could happen in 20 years , it could lose its value , for example
the best way would be a form of a paper wallet , laminated and put into a water,dust etc. proof metal case and stored safely
hardware wallets , usb sticks run a risk of a malfunction more than becoming irrelevant