That is true about mag tape, I suspect tape to be one of the better choices for archival storage. In fact the biggest problem you will have with mag tape is making sure you have a working drive 10 - 20 years in the future when you want to look at your archives. to make things worse tape drives are getting more and more flimsy and fragile as tape tech advances.

my first job we had a vault of ibm reel to reel tapes of old business data. our attitude if we were ever asked to pull any of the data was that we would probably only get one chance at it as the ferric material on on the tape had a disturbing tendency to flake off. note that there are techniques to reduce this, but we did not have the means or motivation to apply them.

And a pedantic observation on your correct point about optical disks. you can't make backups on pressed disks, only recordable ones.