I disagree. One can also want to write more as an end itself, simply to
    become a better writer.

You've contradicted yourself. Writing to become a better writer is a goal that's different than writing as an end in itself. Moreover, writing just for the sake of writing is not a very good way to become a better writer. Using word count goals to improve one's writing is like counting the number of times you kick a ball each day and using that as a measure of your progress as a soccer player.