I think that Disney+ is going to make a lot of people look at the new theatrical releases and really question if they want to spend their money at the box office, or wait for it to be released for streaming. Onward is a movie that, based on the trailers, I don't feel the need to go to when it will most likely be available to stream before the end of the year.
I'm curious to see if that type of thinking will impact other movies with less than billion dollar blockbuster hype like Black Widow.
I think it will affect those "on the fence" movies.
Like, I won't go out an see any of the Disney Live Action re-makes movies in theatres or buy them, knowing I can just watch them 7 months later? (whatever the lag time is, we don't really have a set limit? ) later on D+. I wonder if it will hurt those sales (even though they have been pretty successful thus far) .. and just put them straight to D+ like Lady and the Tramp.
For the big (good) movies, sales will be fine. My family saw Frozen 2 in the theater and we bought the DVD/Blu-Ray/Digital combo pack when it came out so we can watch it again now .. and on future car trips (like next Disney trip).
But .. I know that DISNEY+ has made me stop buying the older classic movies that we don't have yet. Those "middle of the road" movies (from the several Disney dark times (Black Cauldron, Oliver and Company, Dinosaur, Treasure Planet, etc.) .. that the collector in me would have probably bought just "have" and to show the kids, now .. I have no reason to.
Heck Disney+ has been great for to see those Disney the 1940s anthology movies that were often hard to find on DVD. (Watched Fun and Fancy Free over the weekend)
As technology grows .. and streaming in a car is easier (and cheaper) .. Disney DVD sales will probably dry up. For us .. the car is the main reason we have a ton of them (and KEEP them) for now.