If that's your only worry you can update the ink every ten or so years, verify that the newly created paper works, and that should do it. Of course, that's assuming that stays your only worry. I wouldn't convert to metal, at an additional expense if that's your only concern. It doesn't even have to be ten years, it could be less. It really does depend on what you want to do, but changing to metal seems quite extreme if you don't need that. If you think that it could be exposed to certain elements, I'd say go for it.
You should probably have multiple backups in different places anyway, which could actually defeat the point of metal backups, as long as the paper versions are all secured.
Have multiple backups is good and checking usability of backups is good too.
If a wallet on our device is still usable but our backups were damaged and can not be used when we check backups, we can make new backups to replace old ones. If we think backups can be used forever and don't check them regularly, when we need backups for recovery, we can have nightmare.
Multiple backups are good to protect our funds when our device is broken and one of backups is damaged too.