Because lowering the sliders also lowers the quality of the scenery.
No, default service vehicles have their own slider, which doesn't affect the rest of the scenery, just default ground vehicles.
Our own custom service vehicles are instead controlled by the general Scenery Complexity slider, but with GSX, they will be removed automatically from the parking you selected and replaced with the animated ones from GSX, and since GSX will be free to use, that issues will be solved this way.