True. I've seen cromoly frames as light as 3.75 lbs., but I wouldn't bet on their durability.

And that brings up another point - how many of those super light frames will still be on the road in 20 years?

Carbon Fiber and Aluminum can't take deflection, so they either have to make them strong enough to resist deflection, making them heavier... or they have to accept that they have a usable lifespan and will eventually fail. Often catastrophically.

The 4 lb. Cro-Mo frame will still be kickin' it, barring accident or severe neglect.