But if they aren't subbing just to watch that movie, they aren't getting the revenue either way. If someone would go see a movie at the theater and still keep their D+ subscription, Disney would get $10 from the subscription and $5 from the movie watch. If that same person decides not to go to the movies anymore because "Eh, I can wait a few months", they went from collecting $15 off that person to just $10. That $10 was going to come in no matter what if they aren't subbing just to see new movies. Now, like I said, what that doesn't factor in how ad revenue is working, so take that as another large consideration, but it's not about they watch it either way because they don't make money off it either way if the person is not subscribing just to see the new movies coming out to streaming, and wouldn't be subscribed if they had to wait longer.
But given history, even without D+ we know that people would wait, as it would be released on some sort of streaming platform (like Netflix), or VOD rentals / purchases. Heck, back in the day some people would wait for it to show up at Blockbuster, or to maybe buy on VHS or DVD at a store. Of course, it does seem more people are waiting to view on their own terms outside a theatre than ever before, but you can't stop that train now that it has left the station.
So Disney doing this with their own platform ENSURES they are the ones getting the direct consumer dollar, at the price they want, vs a deal with another platform to host their content after the theatrical run.
The landscape has changed, forever. The audience is changing how they watch, it was inevitable.