While M-DISC's marketing is genius in that it makes long term data storage look easy for consumers, the IT industry already has this figured out -- and experts (librarians) already have protocols for this.
1. Choose the best combination of $$/stored-bit and standardization (to ensure future compatability).
2. Store multiple copies in multiple locations.
3. Make sure to copy your data before the equipment becomes a museum piece.
If you were my customer, I would advise you to pick up the latest LTO-7 drive and a box of tapes for around $3k. That's 6TB per tape!
As a cheapskate IT pro my recommendation is slightly altered: Pick up a used LTO-5 drive and a box of new tapes for around USD $800. 1.5TB per tape ...
LTO tapes (standard media) last 15 to 30 years depending on storage conditions and media quality.
While it is tempting to be attracted to Blu-ray because you can picture yourself popping a CDROM from the late 90's in a contemporary Blu-ray drive, it is highly unlikely this form of storage will be available in consumer devices 15 years hence.
If you have even a few TB of data those M-DISC's begin to look both difficult to deal with (they're so small!) and much less economical than LTO.