The 2.5 km mean relative elevation of mid ocean ridges is arguably consistent with a steady state thickness of ≈100 km for the oceanic lithosphere. Here we explore some consequences of this particular value. Based on seismic models of the uppermost oceanic mantle, we infer that no significant heat is added to normal asthenosphere following accretion at spreading centers. Therefore we adopt the view that kinematic thinning moderates the conductive thickening to yield the steady state thickness of ≈100 km. We devise a method for estimating mean rates of thinning directly from topography‐age data, with no assumptions on the detailed dynamics of delamination. We find that delamination delivers a significant fraction (>20%) of the global downwelling discharge. This large non subducting discharge of cold lithospheric material provides an explanation for the most robust features of upper‐mantle tomography, and should be considered in studies of mantle cooling and mixing.