He probably assumed that this would make the quality of Demosthenes’ columns deteriorate, but if it did no one noticed.
There are no more uses of "deteriorate" in the book.
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The situation could quickly deteriorate.
Writing, at its best, is a lonely life. Organizations for writers palliate the writer’s loneliness, but I doubt if they improve his writing. He grows in public stature as he sheds his loneliness and often his work deteriorates.