My understanding is that backup tapes are not even designed to be erased, rather more tapes are purchased as more are needed. Can anyone in IT verify this?
Depends. We reused tapes because the company was cheap. You can get several uses out of them.
I've been in IT for 25 years and have done hundreds of backups. The tapes are not erased. However they are written over when it comes time to use them for a backup. Depending on what kind of backup is done a backup on a tape was usually kept for two months before the tape came up for use again in another daily backup.
In other backups a tape could sit in the library 90 days or over a year until it came up for use. The only time backup tapes were destroyed is when they were bad before use or the tape malfunctioned during a backup. I'm talking about LTO tapes and DDS3 tapes. Files on tapes were never erased! That would quite difficult (time consuming) and was totally against IT policy not to mention completely unnecessary.
BTW, tapes are reused until they fail because they are expensive -- usually 25 dollars plus for a tape. And the tape library has hundreds of tapes. And tapes do last for quite awhile -- usually many years if they are handled correctly.