I was thinking about an UI feature to reminds people that they need to make a new backup.
It would be counter that starts at the keypool size of 100 and decreases every time that a new key is generated. There is a reset button (alternatively, this can be triggered by “Backup wallet…”) to reset it back when a backup is made. It could display a big warning when the number of “safe keys” falls below a certrain threshold (say, 10).
This number could be stored in the wallet itself, or externally (I’m not sure what is best).