This is an excellent, excellent post and it puts into words a certain feeling that I've never been able to sufficiently verbalize.
It's true that the ride system used to be created to suit the material. Look at something like Peter Pan's Flight - they had to invent that ride system specifically to suit that purpose. The Omnimovers weren't bought off the shelf... they had to invent those too.
Nowadays themes are picked to slap on a ride regardless of whether it has meaning to the ride itself or within the surrounding areas as a whole. You're absolutely right in that the parks have become mere containers for individual, franchise-based attractions. There are hundreds of theme parks in the world that do that - Disney's franchises are just more famous. Very few do what Disney used to do, which was create environments and fill them with attractions that would allow guests to live out adventures they couldn't have in the real world.
We're well into the era of "cement lot filled with rides" - that concept is well illustrated by DCA and DSP.