A couple of years ago we had a B767 hit by a runaway tractor. It took out the whole belly skin below the forward cargo bay and it cost around eight million quid to fix it. The nature of the impact was such that it would definitely have taken out the belly of even the toughest composite aircraft. There have been plenty of similar accidents before and there will be many more.
Then there's the possibility of severe structural damage that isn't immediately apparent. Composites may be more damage tolerant than metal but dents in metal are very obvious: even if missed, the slow rate of crack propagation gives ample opportunity to catch it before catastrophic failure occurs. Composite structure tends to let go with a sudden loud twang, as it were, and little advance warning - when it does go, it really, really goes. Also, I can't imagine the autoclave that will take an entire fuselage, so I imagine we are talking about hot-bond repairs. On a large area of damage, a hot bond repair will be a major undertaking with the aircraft out of service for a very long time. And are we going to keep huge amounts of repair material on hand, just in case? It has a relatively short shelf life and costs several arms and legs.
So, the next question is, even if the manufacturers and operators are happy with an all composite fuselage, how do the insurers feel about an aircraft fuselage that would probably be BER in the event of a significant ground incident?
Then there's the wings. Oh yes, of course. They're not a problem because the wings never get damaged in ground accidents.
Ooops!
