In five years, the CD's and DVD's you burned may be corrupt. Optical discs' quality isn't what it used to be. Of course, this varies from manufacturer to manufacturer. If you want to digitally store data, the most future-proof way would probably be to use multiple good quality SD cards.
All types of flash memory are a bad idea for long term storage. They discharge over time.
Indeed. But they discharge extremely slowly. I can't think of a more reliable way to
store data.