Never reusing a card and storing all vacations on one card are perfect examples of the two extremes of madness.
I guess everyone has their methods based on preferences, habits, and budget.
Personally, I end up with 2k+ images after a few hours of roaming around looking for bugs (a lot of focus bracketing sequences), so reusing cards is a must. For now I'm keeping the non-trash images on the computer hard drive, with a backup to an external drive that also sits on my desk. The final stacked/edited images get uploaded to my phone. I'm also starting to think about "investing" in a NAS (network-attached storage) solution that keeps all data doubled in case a drive fails. Those cost about $1k for ~15TB.
Regarding fireproof safes, at least the one I use for storing documents (more of a case than a safe) stipulated in the manual that its purpose is to keep the paper inside from combusting for up to half an hour in direct flame. That implies that it still gets pretty hot in there, so I don't think it'd keep memory cards from degrading and corrupting the data. Perhaps some safes are more insulated than others. If you have critical data that you can't lose, cloud services and multiple backups on devices that are not located on the same premises is probably a safer way to go.
