If you take the example of something like a Crittall steel-framed single-glazed window, a classic of the 1930s, there's just no way to fit double-glazing into the frames without destroying the appearance. The problem of thermal bridging (the metal conducts heat straight through) is also difficult to solve without thickening the frames. Maybe future technology will solve these problems. It seems more likely that the appearance of those buildings will take a hit due to the urgency of saving energy.