Making new models takes time and manpower, and they can only make so many new ones per expansion, among tons of other things they must make. Unless you want the game to look emptier, and many dungeons to give no rewards, they'll have to use old skins mixed with new skins (and we do get a good bunch of new skins).

I don't see why this is so dramatic, or why you think that making hundreds of new models is super easy and takes no time, and you deserve no less. People often sound so entitled and disconnected from the amount of effort that everything we get takes.