My opinion, but the only sure way to preserve these things is through physical media. If, heaven forbid, there was catastrophic event that heavily damaged or destroyed the infrastructure of the internet, all of the "purchased" online digital media would be unavailable or just gone.
"That can't happen!" you say? Maybe, but better safe than sorry.
I enjoy owning the media, I love my 1-10 box set and ST09/STID Blu rays, it's the extras, the cover artwork, I like to see them on my shelf. I've got Netflix for films/TV that a don't really care about as much, hell if I saw a great movie on there that I really loved I'd still get the Blu Ray.
I'm having a similar discussion on an Apple forum about them killing the ipod in favor of streaming services, a lot of people don't get that my 15000 strong itunes library or my 500+ DVDs on my shelf are a reflection of me and my tastes. Having it all at your fingertips is impersonal in my view, but like I said I still use them, but my categorised shelf and music library a personal to me.
Back on topic, I'd love them to do TMP DE on Blu Ray, it's right up there with my favorites, so here's hoping with the 50th coming up.