Having a rendering engine that's really good is useless unless you have the talent to use it to make a movie, and Dreamworks is going to maintain their branding and reputation for a long time regardless of whether their tech is open source or not.
They aren't "giving away the key" because the render engine is not the key. It's a complement, and they're simply commoditizing their complement.
While I agree that high quality art is going to become completely commonplace over the next decade or two, I really don't think it's tied in any way to one company releasing their internal render engine to the public.