I think we need to take a step back here and look at what has actually been said by Infratil. The chief executive has been quoted as stating that they may sell, not that they will. My reading of that is that they are testing the market. By making it known that they may be interested in a sale they are then waiting to see if they get any approaches or offers. The possibility of Glasgow also coming on the market may also be an influence. Infratil will not dispose of Prestwick unless they get a realistic price and now is not a good time for selling UK airports. The BAA's delaying tactics over selling airports is partly in the hope that the market improves and they can get a better price. In the longer term I can see Infratil selling both Prestwick and Manston. They originally planned to own a number of European airports - plans which did not come to fruition. Their only other acquisition was Lubeck which they have already sold. All Infratil's other interests are in Australia & New Zealand so it makes little business sense to continue ownership of 2 UK airports.
Prestwick has survived in the past and there is no reason to think that it doesn't have a role in the future. It doesn't have all its eggs in one basket with very diversified operations from passengers, cargo, maintenance, military, training, general aviation and transit traffic, some of which have increased significantly in recent years. Cargo has been static for some time and passenger figures have declined considerably (here less dependance on Ryanair would help as the only other scheduled carrier is Wizz Air).
Changes are ahead but they are probably not immediate. I am sure Prestwick will weather any storms that come its way.