Describing why the sheath resistance should depend critically on the charge density is both easy (qualitatively) and difficult (quantitatively).
Qualitatively:
Rsh = Vsheath/Isheath (dc)
If the charge density increases, the amount of current carried through the sheath should also increase for a given potential drop since: J(random)@0.25*ni*vi. Therefore, Rsh should decrease as 1/ni (in the coarsest approximation.)
Quantitatively:
Vsheath @ Vfloat (which does not have a simple dependence on the electron density.)
Isheath is space charge limited and does not simply follow ni. The rf fluctuations in the plasma potential determine the sheath average width and Isheath.