All I was trying to point out is that the PSOs have been flown by various airlines over the past two decades and thus they are not the exclusive domain of Aer Arann. Being locally based allows Aer Arann to offer the best deal in a tender, but if no local airline is around, other airlines have a bigger incentive to bring forward a proposal as they become more competitive. For example, airbaltic has tendered for domestic PSOs in Germany in the past, and I seem to recall that VLM also tendered for some Irish routes.
I agree that the whole situation is not getting easier because more and more suitable aircraft and airlines disappear. In the end, however, it all boils down on how much government is willing to spend on a PSO contract. If the deal is attractive enough, there will always be a provider.