Storage media fail. Whether it's a hard drive, DVD or an USB stick they all sooner or later fail. If all your backup drives containing your encryption key die, you are in trouble. Hopefully, there is paper to the rescue. It won't replace backing up to external storage, but when all else fails, at least you have a chance to avoid losing your Bitcoins.
Here is a link to this program: http://ollydbg.de/Paperbak/
It's open source, so you can port it to Android, iOS, macOS and Linux rather easily.
It also includes encryption, but don't rely on it. Encrypt your wallet in your Bitcoin program and GPG it too, so it ends up triply encrypted
Download many copies of the tool too (including source code) and put it on your cloud and your storage, so even if it disappears from the Internet, you wouldn't end up with the fancy notepad paper.
Make sure that you can restore your backup before you can rely on it.
Here is a link to this program: http://ollydbg.de/Paperbak/
It's open source, so you can port it to Android, iOS, macOS and Linux rather easily.
It also includes encryption, but don't rely on it. Encrypt your wallet in your Bitcoin program and GPG it too, so it ends up triply encrypted
Download many copies of the tool too (including source code) and put it on your cloud and your storage, so even if it disappears from the Internet, you wouldn't end up with the fancy notepad paper.
Make sure that you can restore your backup before you can rely on it.