To begin with, the following assumption is false:
>To write well you have to think clearly, and thinking clearly is hard.
For most people, most life situations which require clear thinking have nothing to do with writing.
>This is why eminent professors often turn out to have resorted to plagiarism.
What's the percentage of such professors ? In the university I studied, there is no case of plagiarism till today. And plagiarism is not done because professors can't write, but due to other professional factors.
>If you're thinking without writing, you only think you're thinking.
As if writing is the only way to think well/correctly/effectively. My father never wrote a word: still, some of the most thoughtful statements I ever heard in my life were told to me by him during our conversations.
When you face a situation of danger, such as a wolf is running towards you: will you start to write your thoughts about what you should do, or will you just run right away and decide about the safest paths to follow while you are escaping ?