Dave,

your argument makes sense, but is it not the case that when presented with a rider profile for which you would choose a heavier guage chainstay in steel (say 1.0 mm instead of 0.7 mm) you could use a heavier guage Ti tube (say 1.6mm wall rather than 0.9). The Ti tube will still be something like 20% less stiff than the steel one but when combined with the higher stiffness of the other Ti main frame tubes (due to their larger diameters) the overall stiffness would be OK.