The only way an external drive wears down while not in use is excessive heat, moisture or strong magnetic fields.
I store all backups for the whole company in a safe on site. (plus one set off site). We do that for now over 15 years without the slightest problem. (Granted that's a safe you couldn't even move with a fork lift
I personally don't trust encryption since I'm not convinced that the backup would be usable in each and every odd situation (say your hardware totally changes because the building burned down, or for some reason a bit on the HDD flipped in a critical area etc.) That data is vital for the existence of the company and I'm not willing to take any chances.
For less important stuff it'll be fine to have one set stored away and perhaps another set in grandma's dresser