I am sure the PSOs are the least to worry about. Those routes would be tendered out again and usually there are a few airlines that are interested, often also from abroad, to pick up the pieces another carrier has left behind. Just look at Euroceltic or the situation when Cityjet did not start the NOC service.
The bigger question is how long those routes meet the requirements for state subsidies if surface travel has improved - or how long the government is willing to fund such routes if passenger numbers decrease.