That's not a mistake. Disney/MGM was supposed to be a working studio. The entertainment consisted of touring the studio's, lots, sets, sound effect shows, green screen TV production, etc. Lots of things and the ride idea was not really important, it was, like EPCOT, something different, but also copied after what Universal already had in Hollywood and were building in Orlando. It's todays guest that feels that it need rides, rides and more rides and it does, but, that is because once they closed the studio part, there wasn't much left to do. It's about that time that ToT and RcRC showed up. Previous to that the whole theme of the park was how entertainment was created. Everything from Indiana Jones, to TV, to back stage and shows. The backlot tour with the walking tour of sound stages took hours all by itself. Rides were not necessary or needed.
The studio theme didn't go bad because people didn't go to it, it went because it was a flop as a working studio as far as Disney was concerned. It was highly infused with CM's and the money from movie making had dried up and it could no longer support the staff needed to keep the studio facade going. So Disney shut it down and all of a sudden there wasn't much to do. It still had some shows, like Indy and the Muppets and Sounds Dangerous, for what it was worth. Then along came "Do you want to be a Millionaire" and that was popular for a brief time period, but, it ran it's course. ToT, RcRC and TSM took up some of the slack, but not enough. But, DHS was never designed or planned to be a ride heavy thing, and if the studio thing had worked it still wouldn't need it.