Optical disks are simply "removable" digital storage media varying only by capacity. Digital content can be stored on multiple terabyte spinning rust just as easily.
Please explain what you mean by your statement that digital content “can be” stored on a hard drive. Do you have Blu-ray and Ultra HD Blu-ray surround-sound audio/video loaded on your hard disc drive, with no degradation in quality? What about SACDs featuring surround-sound? If so, how did you get the content loaded onto the hard disc, while preserving all of the features and quality that the disc provided? What software did you use? How much effort was required to install and configure the software? How much effort was required to load a Blu-ray movie, or SACD?
Let’s use this Blu-ray box set as an example:
Rafael Frühbeck de Burgos Danish NSO
Ludwig van Beethoven: Symphonies Nos. 1–9
Joaquín Rodrigo: Concierto de Aranjuez
Hector Berlioz: Symphonie fantastique, Op. 14
Richard Strauss: Eine Alpensinfonie (An Alpine Symphony), Op. 64, TrV 233
These Blu-ray discs feature 5.1 DTS-HD Master Audio, and HD video - and they deliver excellent audio and video quality. (And this is just one example of many classical, opera, and ballet recordings available on Blu-ray.)
Can I buy this box set as a download – with full-quality DTS-HD Master Audio 5.1 surround-sound audio, and full-quality video? If so, please provide a link. How long would it take to download?
OTOH, if the content must be copied from the Blu-ray discs, how would you get the content onto a hard disc drive? How much effort? How much time? I have read about copying Blu-ray (and SACD), and I learned a new word: “faff”.
My limited experience is that metadata is an inconsistent mess for classical music, and often requires editing. (I have a download of Beethoven Symphony 9 wherein the “artist” is listed as the soprano - not the composer. What about conductor? Orchestra? Soloists?)
How are multiple movements of a classical composition concatenated - in the right order?
Suppose for the sake of argument I was able to get the content of all of my Blu-ray and SACD discs installed on a hard disc drive, with no degradation in quality, with suitable metadata, and multiple movements properly concatenated. How would I play this through my vintage tube amps in uncompromised 5.1 DTS-HD Master Audio? What DAC will handle all of these formats in 5.1? (I refuse to listen to classical music with a solid-state AVR in the equipment chain.) How much does such a DAC cost?
This box set includes 12 major classical works on 3 Blu-ray discs that provide the convenience of dropping a disc in the tray and hitting the Play button. Disc #1 of this box set contains Beethoven Symphonies 1 - 4; that’s a lot of music on one disc before I have to walk over to the Oppo player and change discs. To me this is not a problem worth solving.
The future may not favor discs, but I want to enjoy music today.
Copying classical SACD and Blu-ray discs to a hard drive does not appear to be a practicable answer (except perhaps for PC hobbyists who are willing to invest significant effort).
If in the future there is a reliable streaming service that provides all of the content I want with hi-res audio/video featuring surround-sound, then I might consider it. (I.e., no smartphone/tablet apps that freeze or loose synchronization, no degradation in audio or video quality, and no drop-puts.) Based on my experience with Spotify Premium and Netflix streaming, they’re a long way from equaling the quality, reliability, ease-of-use, and content offered by Blu-ray, Ultra HD Blu-ray, and SACD discs – at least for the classical music that I enjoy.
I’m still waiting to discover a machine other than the Oppo UDP-205 that does everything I want. Therefore, I’m taking delivery of another UDP-205 tomorrow.
As I’ve said before, everyone’s situation is different. If you’re consuming recorded music as audio-only (i.e., no video), in stereo only (i.e., no surround-sound), and limited to CD audio quality or lower (i.e., not high-resolution), then your circumstances are different from mine. To each their own.