I wish that were true.

Alas, M-Disks are hugely more expensive than magnetic tapes, even when not taking into account their much greater volume.

The biggest M-Disks are of 100 GB and the cheapest that I see on Amazon (in 25-piece sets) are at about $9 without taxes and shipping. At that price, I can buy not 0.1 TB, but 2 TB of magnetic tapes, which also have a much lower volume, at 18 TB per tape for the older and cheaper LTO-9 cartridges. I can easily carry with me a suitcase with 360 TB of data (20 LTO-9 cartridges), or store it in a safe place. Carrying 3600 M-Disks or storing them in my home would be impossible.

M-Disks may be used only to archive some small amounts of more valuable data, e.g. documents or programs that you have written yourself, a few scanned legal documents, a few more important books, a small amount of music that you prefer, a few family photographs.

For bulk data, if you cannot afford the cost of tape drives, the only alternative is HDDs, but with HDDs you must take care to always use both duplicate HDDs and archive files with redundancy, which allow the regeneration of their content when any not too large part of the file becomes corrupted. You must also take care to migrate the data periodically to new HDDs, after a few years.

Due to their small size I have not tried to use M-Disks until now, so I do not know if my older Blu-ray recorder can write them. There is also the problem that the availability in the future of Blu-ray recorders and readers is uncertain.

From prices I can see it's 10-20x more expensive than tape archives.

M-Disks are not cheaper than tape for bulk data storage. Once you're doing enough of this that you're not limited by the cost of the drives, it's about a factor of 25: $150/TB for M-disk vs $6/TB for tape.

Bluray is cheaper per unit, but not per terabyte.

But will there be any bluray drives available in the future?

Which LTO are you comparing with? Locally (Spain) it seems I get similar EUR/GB for BlueRay M-Disks as LTO-8, but LTO-7 and LTO-6 is way cheaper than BD. What exactly are you comparing here?