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I think that statistical analysis has demonstrated that offense is primarily a series of independent contributes, but the issue is still up in the air for defense. If each defensive contribution were primarily independent, then there shouldn't be any diminishing returns. If you improve your shortstop, how good or bad the third baseman is has no impact on the upgrade from Yuni to Jack Wilson. If, however, the zone of influence of the third baseman and the shortstop overlap significantly, then the fear of diminishing returns certainly applies.
I'm agnostic on the subject.