Originality does not require one to end all discussion. A.J.P. Taylor's book generated a tremendous controversy and left a large number of unresolved issues. The point is that even in an overworked area like the origins of world War II, there are plenty of important issues which need to be analyzed further. Moreover, an analyst should take comfort in the fact that disagreement among scholars is both healthy and necessary. An original argument must therefore also require primary research.
Primary research is part of the process of writing.
While one should always start one's research with a question in mind, it is more than likely that that question will change in the course of investigation and analysis. There should be a constant and vigorous interaction between the research and the writing of a major essay. Put simply, the absence of primary research suggests that the essay is not rooted in evidence, but rather based on an interpretation of others' interpretations. That outcome is unacceptable, and one that is avoided only by primary research. To be persuasive, an essay has to be well-written.