The process of chemical reaction is complicated, which is not as simple as A+B = C. Hydrogen ions of concentrated sulfuric acid first turn ethanol into positive ions (molten salt), and then remove a molecule of water to generate ethyl positive ions, which then spontaneously remove a hydrogen ion to become ethylene. Note that hydrogen ions are borrowed from sulfuric acid to participate in the reaction, and finally return to sulfuric acid.
In fact, sulfuric acid in this reaction is still not a catalyst in the classical definition. As long as the classical catalyst exists, it should be able to promote the reaction all the time. Adding more and adding less only affects the reaction speed. As for why you think for yourself.