They have to include them anyway, because almost no game is sold exclusively on Steam. The only features they don't have to include are features for the customer, so that's not relevant. The features relevant to developers are access to market (which is the same as physical retail) and DRM, which they have to include anyway if it's available outside of Steam. Just for reference though, Denuvo is about $100k/y, which is close to the cost of 1 employee.