Cost for M-DISC and the ability to use them in the future is no different than the 'HARDDRIVE' you think you can maintain into the future. You can't maintain either of them beyond practical use. The issue is which will still be there when you need it.
Lets talk about computer hard drives.
In my life I have seen
Channel Drives
ATA
Microchannel
Scsi
Fiberchannel
Serial ATA
SAS
ATAPI
and that is not even touching the DIGITAL Drives. In 6 decades there have been at least an average of 1 major change every year.
Now lets look at CD, DVD and Blueray....NO CHANGE because they are ISO Standards. They do not change, they are added to, there are new standards but the old ones stay the same, forever.
I have not even touched on the interfaces for drives. ATA, SAS, SCSI, USB, MicroUSB, USB-C, USB-3, ThunderBolt, do I really need to go on because that was in the last 5 years of change. A hard drive from 10 years ago unless you have the OLD equivalent to run them on wont work. The OS may not even recognize it.
M-DISC, Current version of DVD drive plug it in....works. Game ovre M-DISC wins in the long run, BUT......at some point M-DISC will be outdated, which will be well past ANY hard drive you buy today. Not just the hard drive but the format, os compatibility etc. So before your M-DISC is no longer supported you convert it to the NEXT format of long term storage which, as progressive technology theory has shown will be MULTIPLES of times bigger and MANY times cheaper to use...and it will probably last 10,000 years by then,
The point is, hard drives are great but I have at least 30 in my collection that will never be off loaded again because there is no hardware to plug them into.