The biggest existential threat to any civilization is its own state, surprisingly absent from your list. A further and more generalized abstraction is that the

The biggest existential threat to any civilization is its own state, surprisingly absent from your list. A further and more generalized abstraction is that the biggest threat to any civilization are self-interested power centers of any kind. Those can take the form of states, corporations, unions, billionaires, etc. Any entity that begins to operate in its own interest and has vastly disproportionate power when compared to the individuals of which the civilization is made.