The reasoning for the appearance change is given canonical explanation. The Elites, for example, are from the Hesduros colony world. Having been separated from the main Sangheili population for as long as they have, appearances began to change. If you don't like the appearance of the new enemy models, then tough. Frankly, there's nothing you or I can do about it. Also, yes, I know that the Halo 4 style of Elites are being used for the Arbiter's faction in Halo 5. My best guess on why they did this is to save on resources, because otherwise they would have needed to create completely new models for NPCs that only show up for a portion of the campaign, which is neither quick nor cheap.