Oh, much smaller diameter tube for a chainstay, makes any given damage, much more severe. Top tube is lower stress, larger diameter. For top tube, in bending, tension, compression, lots of redundant load path around that soft spot I think. More critical I think is when standing on the pedals and pulling hard on the handlebars, both down and top tubes in torsion, whether there will be any localized buckling at the soft spot, or expansion of it due to fully-reversing shear stress at 45 degrees to the long axis.