For new developments, absolutely, but once something has been out for decades (plural), modifying and selling that isn't so unethical. Keep in mind that Turtle WoW built off and supported that original game client from over 20 years ago, they weren't just backporting a bunch of content from Blizzard's newer releases. It's the same as if someone wanted to sell a modded Super Nintendo game, society would not be worse off for this.