This is exactly the point that both private business and governments don't like to talk about re privatisation of services that have been traditionally provided by governments. In the past decades these have included hospitals, prisons and, in aviation, airports and ANSPs. There already instances wherein a private operator went bankrupt or was otherwise unable financially to continue operating (a failed experiment in Texas re privatised prisons comes to mind). They key, to me at least, is the question of privatisation of "critical infrastructure'. Meaning those services that, if non-functional, have an immediate, significant and unacceptable effect on the country or society. Obviously prisons, and most hospitals, are examples of institutions or services that must continue to operate at some level. I.e. government cannot simply say, "We'll just let it close and grow mouldy." Clearly there are examples in aviation: A particular airport or airport control tower could close unexpectedly and not result in a national crisis. But an ACC? Or an international airport (there are countries that have privatised their only international airport -- a capital city). If that operator ceases operations one morning, what then? When that very question is brought up at discussions about privatisation of "critical infrastructure" the question is either met with blank stares and silence, or someone says "Well, the government would have to immediately step in, wouldn't they?". Yes, they would - if they can.