For the record, I don't "slag" on aluminum or carbon. I don't expect to see carbon bikes pressed into extended, utilitarian service when they are decades old, for the reasons stated above, unlike the current situation with vintage steel bikes of all types.
I have stated my opinion in other threads, based on metallurgical theory (which has been well elucidated above, thank-you), that an aluminum frame or fork is more likely to experience a fatigue failure than a steel equivalent. To the extent this has been mitigated by "overdesigning" the structure for fatigue, the weight and ride quality of the aluminum bike has been compromised.

But I agree, carbon and aluminum certainly have their place in the bike world, however that place isn't as a desirable alternative to a steel framed bike for rugged, utilitarian, unlimited service.