Depends on who's asking. Amazon Glacier never formally disclosed their storage medium (at least as of a few years ago) and one of the theories on what it might be was actually a robotic optical disc changer library based on BD-XL, and the cost/capacity actually does math out. Yeah, discs might be $15 a pop (for a quad layer/128GB disc) for you as a consumer, but when you're Amazon and you'll be buying the complete output of at least one optical disc factory, the economies of scale kick in. It's just expensive because there's no market for 128GB media for consumers (and honestly these days hardly any market for WORM media at all as a consumer), it's not inherently that expensive to make the discs.

(I believe the final consensus pointed to arrays of HDDs where most of them are powered off, and the number of "live" drives per rack is bounded to allow high density/low cost, hence the need for access time/service level bounds, but the BD-XL idea is still intriguing!)

With the consumer discs, even considering cost per GB, the amount of effort required to handle a large library of low-capacity discs is just too great even if the cost is a little bit better. 128GB discs would have been very usable 5 years ago but again, those discs were never affordable to consumers, and the 25GB was still some effort at that time. Today even 128GB is not all that much, as data has grown. As far as I know there is nothing realistic on the horizon to replace blu-ray with higher capacity either, if movie content started being released in 8K it probably would be something like BD-XL with AV1 encoding (or maybe H265 again), not a fundamentally new iteration like DVD->BD.

The future for consumer storage seems to be SSDs and hard drives for fast and slow/bulk storage, and cloud for nearline storage. Tape is still relevant for enterprises though especially in automatic libraries.

Interesting, i didn't know about glacier.

The theory of hard drives being shut off/ powered on dynamically in a rack sounds intriguing. Sounds simple and yet difficult because of the rare usecase, i.e. no commodity hardware available. Maybe something to test out for colo backups to keep power usage down and prolong disk health.

I believe that commodity hardware is sort of available. Look at MAID hardware. It's aimed at enterprise customers so it's pretty expensive if you buy new but there's probably older stuff on ebay or a reseller site.

Capacity per disc too. Blu-Ray discs tops at 125GB and there’s no cheap and easy way to automate disc handling to work around that.