The thing is, you're the exception. Not many people are going to watch BD movies on their PC.
Definitely. It's pretty clear that most people are perfectly happy with Netflix - or even lower quality streaming services - for their main video watching rather than DVDs or BDs. If I had multiple PCs, I'd also be perfectly happy without an optical drive; my M1 holds my only machine, so it has to do everything (daily driver workstation, media playback, portable VR demo rig, etc).
That's not to say that 5.25" bays don't have other uses, of course. But again, despite their use, most people these days don't need them and will leave them sitting empty.
5.25" bays can definitely go for SFF, and most ATX rigs can probably make do with slimline drives too. The minimal speed and noise tradeoff for slimline drives is acceptable there. I'd argue that HTPCs don't
generally need to be SFF, as anyone who is sticking with 'legacy' optical media for quality likely has some sort of AV stack with a receiver/amp (or a receiver + one or more separate amps) so a slightly bloated chassis isn't too much of a problem in amongst everything else.
Overflowing in space or in bitrate, because both are solveable
I don't know if you've seen my NAS build in the casemod subforum, but it's a 6-disk 2TB NAS with a 2-disk redundancy (so about 7,5TB of capacity) and it allows >100MB/s read/write. A good NAS is a blessing and will solve part of your problem.
Space. I have a 5-drive Microserver with 9TB of usable capacity (RAIDZ2 for failure tolerance). I just have too many BDs, as well as the NAS storing backups and other data. I'd sure like to have the cash to blow on a nice 3U 16 drive backplaned box (and the drives to fill it), but then I'd probably have other problems.
That's what I mean about ever-lasting formats. Physical containers are reliant on proprietary physical devices to play them, non-physical formats are not.
True, continuous transfer of data to new formats and media is better than sticking it on a disc/tape/disk and hoping it doesn't rot. Unless you have a massive tape archive and can pay to keep the tape robot and drives running for decades (e.g. CERN).
But every single format shift so far has demonstrated that when storage becomes cheap enough to copy all my optical media onto a central store without quality loss, then a new media format with higher bitrate requirements already will be there and would flood that central store. Not only is UHD ('4K' and '8K') on the horizon, VR is prompting a resurgence in panoramic video (various methods of packing images into what is effective a 16K frame for a reasonably acceptable angular resolution) for just flat images, and beyond that Lightfeild images and video which can easily run into multiple TB at high compression ratios.