"Additionally the BBC's high-quality disc extras do not seem to have made the jump from disc to digital, signifying possible further decline for 'value added' features such as commentaries and documentaries in the future."
Between availability, audiovisual quality, lack of extras, and packaging I think that physical media will remain the premium choice for a long time to come. Add in the fact that there is little to no availability of 3D content on streaming services, or where there is, it's only through a handful of devices (not roku, chromecast, etc.) and you've got very good reasons for physical media to be an attractive option that enthusiasts will happily pay for.
But then again, my DVD and Blu-Ray movie collection has now exceeded 500 discs - all legitimately purchased, so maybe I'm biased. But, the ads proclaiming "own it on Blu-Ray or DVD today!" is attractive to me, rather than the fucktarded "you only license it" nature of streamed media. I bought these DVDs and Blu-Ray discs, and despite what revisionists claim, I OWN those copies... so if Netflix, Amazon Prime, or in this case, the BBC decides to not host the content any more, I still have possession of my non-revocable legally-owned copies.