First thing, the old .DAZ scene files only store the rig, poses, dial settings, render settings, surface settings and which of the several thousand DSO/DSD/DSV files goes where, these are generated when you saved the scene, lose that data folder with all of those files and your .DAZ files are useless.

.DSI are Layered Image Editor (LIE) XML code files from DS2 and 3, the LIE in DS4 was changed and no longer creates those files but can still use any existing files.

Now depending on which product you originally used in the scene, it might be a case of it's been updated and those files removed from the product.