Upper panel: Boyarsky et al. (2018): B18 surface brightness expectations for an NFW DM profile of the Galaxy. Portail et al. (2017): P17 DM profile using B18 decay time of sterile neutrinos. The red data point shows the best-fit value with uncertainties from this work. The shaded area indicates where the DM mass models and X-ray measurements become more uncertain. Lower panel: as in upper plot, but with data points showing expectations for eROSITA extrapolating from the B18 measurements assuming the errors scale with the root of the relative observation time i.e. that the grasp and background of XMM and eROSITA at 3.5 keV are the same. As an all-sky survey, eROSITA will either accurately measure or rule out the 3.5 keV line from the wealth of observations far from the Galactic centre where the astrophysical backgrounds are low.