I could re-express someone else's thought half a dozen times, never infringing on their copyright. Some rewrites might be more elegant or effective. My words might be those that carry their idea to a broad audience. Their words have died. But their idea lives on. They just don't get any credit for it. If the original ideas are good, they can successfully transcend the original means of expression -- to the detriment of the original thinker's ability to profit from them.
With rare exceptions, a significant idea exists independently of the medium in which it is expressed. But an inconsequential idea is dependent on its medium of expression. The harder it is to separate an idea from its expression, the less significant is the idea. No matter how many volumes you write about an
inconsequential idea, you cannot turn it into something important.
Copyright protects only your expression of the idea, whether it was your idea to start with or not. It does not protect the original thought. A transformative idea is inherently likely to be purloined and proliferated;
yet original thinkers continue to publish their work. If they did not, the world would be a much poorer place.