A freighter version for the -400 has potential because the workload to produce that is "relatively" light.  The -500, being a shorter variant would require a much more laborious effort to change both the 3D and the flight model, being geometrically disparate...and the return on investment in that, I believe, probably would not justify the efforts.   While I think a freighter variant would appeal to a statistically smaller set of customer use cases, I recognize that there is still a place for it.
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