I'm not sure why the OP isn't considering USB external hard drives for backup purposes, because HDDs have a much better lifespan than the flash that you can find in a USB pen drive.
Probably the price. He is trying to find the cheapest possible solution. The problem is that if you want to save money on cheap hardware, you'll pay for it elsewhere.
Aside from scratch, finding device to read/play it shouldn't be hard since you could use any DVD/BD reader. I expect few company will continue to make DVD/BD reader in the future.
CDs, DVDs, and Blu-rays will suffer the same fate as cassette tapes. They will become obsolete. Modern laptops are already sold without such optical media. So, in the future, you'll have to buy a separate device just to run the discs. Compared to that, USB ports are much more represented and used for all kinds of things (at least for now).