Tapes hold up really well if they're not in absolutely awful storage conditions. And the claim at least was that the early CD-ROMs were quite durable, being a straight up laser carved diffraction grating. CDRs on the other hand rely on dye which will degrade rapidly.
M discs were made, quite possibly to meet the Mormon need of post Armageddon lineage documentation, to last at a minimum of 1000 years in reasonable condition. They are just special, expensive CDRs that use a different dye system that won't break down in 4 years.
My optical disk drive was $17 and has M disc writing compatibility, and my understanding is they are meant to be read by any CD reader.
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.
You or I can't make backups on pressed disks, but it is an interesting consideration if part of your archive consists of commercially released movies, games, music, etc. that could be stored as original Blu-Rays, DVDs or CDs.
