I imagine that Disney would take issue with SD if material that Disney owned the copyright to was used in SD. They would sue. SD would have to be taken off the market.
Thus, Disney has the power to ensure that their copyrighted material remains protected from outside interests, and they can still create unique things that bring in audiences.
Any small-time artist that produces something unique will find their material eaten up by SD in time, and then, because of the sheer number of people using SD, that original material will soon have companions that are like it because they are based on it in some form. Then, the original won't be as unique.
Anyone using SD will not, by definition, be creating anything unique.
And when it comes to art, music, photography, and movies, uniqueness is the best selling point; once something is not unique, it becomes worth less because something like it could be gotten somewhere else.
SD still has the power to devalue original work; it just gives normal people that power on top of giving it to the big companies, while the original works of big companies remain safe because of their armies of lawyers.