when they're powered down.
There's no periodic cell refresh in flash memory like there is in DRAM. When USB sticks are plugged in, all you are doing is powering up the flash chip and interface ICs.
You'd have to read a block then write it back to actually refresh the stored charges in the cells.
I've got photos in Flickr dating from 1999 onwards. Ten thousand or so of them, and a couple of the early ones are now corrupted.
But they are my "other backup" for Google photos so I don't mind too much. I also have a USB Blu-ray drive at home that I use to periodically burn M-Discs that I hand out to a few relatives.
That's about as good as I can conveniently do for backup, and it's probably better than the single-point-of-failure box of negatives that my parents have in their cupboard.