I really don't see them destroying Front Lot+ Studio 1. They thought about it but decided not to. You may not remember but Buena Vista Street reused some of the existing showbuilding of DCA 1.0. Besides, you can't expect them to bulldoze everything and start anew when they didn't even have the budget to demolish the Dinotopia scene from WDS Studio Tram Tour...
Some elements of this expansion are, indeed, placeholders to maintain a decent capacity in this park. The Stunt show Arena, Cars Road Trip and the newly revealed PIXAR Plaza will likely stay 'til Pandora opens (2029 ?). A new show will be introduced next year in the Stunt Show Arena. That's gonna be fun, but let's face it DLP won't maintain such a stadium 20 more years given how Avengers Campus has been successful so far and how costly it is for TDP to keep it. Cars Road Trip occupies a surface area of an entire land while it *only* cost them $40 million, $20 million less than Star Tours 2.0. Finally, the Costuming/ImagiNation building is getting less and less relevant. Basically each land of the WDS will benefit from a dedicated building for performers, a good portion of the current costuming building is now part of the Ratatouille area (Public restrooms and the Marianne shop), its days may be numbered even more now given how important the now under construction costuming building (Dreams) will be. It wouldn't surprise me much if they decide in the 2030's to transform the ImagiNation building into a Pixar counter service restaurant with a "Worlds of Pixar" store alongside a more elaborate Meet&Greet area.