That's probably the best way to gauge how "essential" extras are (well, I mean besides their name, which denotes their status pretty clearly)
If the extras on any given blu-ray/dvd were packaged separately and sold for something like 5-10 bucks next to a barebones release of the film, would most people bother to pick up the extras? Like if I'm headed to the store, and I know I've got 25 bucks in my pocket - if I saw the blu for 15, and the extras for 10, would I actually snatch up both, or would I just buy the blu and get myself a six-pack on the way home?
Let's use the Lord of the Rings example, maybe the best bonus features package in the history of DVD (although I'd argue both Blade Runner and the Alien sets outstrip it, but just barely)
If they had packaged those documentaries on their own two-disc set, and titled "The History of Middle-Earth" or something like that, and released them on their own, what percentage of Lord of the Rings fan do you think would cop those?
I'd bet something like 30-40% at best.
Not only has netflix/streaming basically eliminated the desire for them, but due to YouTube housing most of the good stuff within weeks of it being released, it's starting to seem like this stuff (which is largely promotional in the first place) is being seen as a thing that belongs on YouTube for free anyway.