The fatigue life of aluminum depends on the loading applied to it, and you can design for any desired life expectancy, so that isn't really an issue. As far as that goes, it's usually spokes that fail, not rims.
Theoretically, there's no reason you couldn't make carbon wheels to support a 1,000 lb rider. If there's an issue, it's likely to be because anything new and light was likely made as light as possible for racing applications. Ang that would be an issue regardless of whether the product in question was carbon, titanium, or what.
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