An interesting point not touched upon is that the types of people using USB sticks has changed. Because the use of technology filters down from tech savvy, to general population, to people late to the scene or can’t change.
We are in that last stage now. They are buying by price and so easier to take advantage of.
Mould eats the fibres and dye, so it might be a permanent change I’m afraid. There are specialist mould removers but they might bleach coloured fabric.