You forgot the third leg of the stool: strength. A high tensile steel frame could be made that would be as stiff and light as one made from superalloy. Obviously the rider would have to be very light, very smooth, ride on smooth surfaces and not be a strong pedaler to safely stay within this frame's service envelope, and since material cost is only one part of the total cost of fabricating a frame (especially a very light one) building a flyweight high tensile frame would hardly be worth the trouble.

tcs