Something to consider is how many people are spending $60 a year on Disney movies right now? Of which Disney gets a smaller cut, splitting it with theaters, stores, production and transportation of physical media... Then you can stop buying Disney movies at any time, but you get to keep any Bluray/DVDs you already have. Not that way with D+, you're paying Disney directly and when you stop it's gone.
Sure people are going to buy less physical media, but that is the existing trend anyway. Movie theaters are more about the experience and seeing it early. They've also been in the decline and may take a smaller hit, but will be around for a while for the night out. Stores dedicated to physical media are going out of business, and movie sections have been shrinking or replaced with something that does sell.
It also makes sense for Disney to use The Simpsons to sell their new product, Fox has been doing it for years. Like how Viacom/CBS uses Star Trek. The Simpsons was a sales point for Hulu+.