There seems to be a presumption about that steel as an engineering material is no longer being developed. That is not entirely the case. While the elasic modulus of steel is pretty hard to bump up, it's "strength" properties such as yield strength and ultimate tensile strength are still the subject of ongoing development. Some of the more exotic "superalloys" (whether they are steels or not is debatable) and the maraging class of multiphase steels have specific mechanical properties rivalling those of the more familiar exotics. In fact the maraging class of steels has specific mechanical properties that place it in the restricted technology category due to its specific mechanical properties as regards sensitive technologies useful for centrifigal fissionable isotope seperation of uranium alongside some of the more familiar exotics.
Another possible development that could make ferrous based materials competitive with other exotics is the use of steel as a matrix material for metal matrix composites or MMCs. MMCs are commonly thought of as Al based materials, commonly with isotropic (having equal properties in all directions, essentially) ceramic reinforcement, but using preformed anisotropic (differing properties in different directions simply put) inserts can address modulus of elasticity and strength issues by putting reinforcements of exactly the sort required by a specific application. As well isotropic steel matrix MMCs with very impressive properties using titanium boride reinforcement are under development in Japan.
The relatively high density of ferrous alloys and the subsequent need for extra thin sections to achieve comparable structural weights to other advanced materials has raised legitimate concerns with buckling or "oil canning" failure modes for them, but still I would argue it to be premature to write off steel or some similar material as a "dead horse" in ultralight bicycle frames.
I'm not saying robust superlight ferrous based or steel matrix MMC frames will ever be produced, just that the possibility still exists. We don't know where the technology might lead.
Kurt "still riding that dead horse" Sperry Bellingham WA