It is incredibly tough for Universal to find something that can fit inside a stage show, but it's not impossible. In the short term, they have IPs they can utilize for a short-term show (Sing, DreamWorks, Beetlejuice, etc.)
I also don't expect all empty venues to be filled with shows. For IOA, Sinbad should be saved for whatever happens to Lost Continent, but Toon Lagoon could feature something. That side of the park needs something that doesn't involve getting wet or standing in line.
I'm sure Universal wished they had built a theater in Seuss Landing instead.