Who would have thought that two USB sticks would stop working on their own at the same time? I assume it's the same manufacturer who clearly did a very bad job - because I personally have USB sticks that are functional for over 15 years, similar to SD cards that still work.

The lesson of the story is that any electronic backup cannot be considered safe in the long term - and if someone is already going in that direction, I recommend that they don't save money and buy a USB stick that has high manufacturing standards and thus can save data much longer than these cheap devices.