Carbon road fork lifespan

No. Just common knowledge amongst cyclists and mechanics really.

Wouldn't manufacturers mention it on the packaging if there was any sort of failure likely, or face enormous lawsuits?

I don't think failure is likely. They recommend inspection by a mechanic I think.

How often do they replace aluminium aircraft?

Wat? They'll replace each component at predetermined intervals deemed safe. Just like you and your bike but probably a bit better organised :-)