I agree. The sets are stylized and I'm glad they have a stage-built feel to them. And I wish more movies would do that, as it does indeed help performance.
However, I still think there could have been more detail in the Grail Temple. A floor mural, for example, would not have hindered the ground breakup later. Or some carvings in the walls besides the statues. Maybe paintings of religious import on the walls. Something. Just flat rock wall didn't do it for me, considering how richly detailed every other set in the Indiana Jones world has been. And the stage-lit ceiling was too much of a cheat. They could have had shafts of light coming in, implying the sunlight streaming through the openings at Petra, at least. One shaft could have fallen on the steps between the lions, so when Indy went alone, it was like God was watching him. Or maybe, as the sun shifted, that light could have moved to Jones Sr. when the Grail was brought to him. Anyway, the Grail Temple set was really flat in every way -- No depth, no detail, no interesting lighting, nothing. Not worthy of Indy's finale (or what was presumed to be his finale).