With potentially the two Glasgow airports for sale interesting times are ahead for aviation in the west of Scotland.
However as I suggested in the Edinburgh thread (and was promptly shot down) if Ryanair were to move to Glasgow, then Prestwick may actually benefit in the long run. Allow me to explain my thinking, it may be wrong or you may not agree, it is simply my opinion.
Ryanair has a total stranglehold over management and facilities at Prestwick. With the fees (if any) they pay, Prestwick will not be able to gain much of a return on any investment that is made in the Airport.
This we all know, but look at it from another point of view, from a prospective airline, they see that Ryanair has a stanglehold over the Prestwick market, they know that Ryanair will likely try to crush them with agressive marketing and low fares. This monopoly makes Prestwick (along with the severly out-dated facilities etc) an unattractive proposition for an operator looking to come into the Glasgow market.
However if Ryanair were to decamp up the M77, new markets could be targeted by PIK management (again, this is opinion) such as E-Jet or CRJ size operators for some of the stronger performing routes or some city routes with short break potential (Paris, Barcelona, Budapest etc) along with a tour operator or someone like Monarch for sunshine routes.
However I have to concur with theredbaron, in this financial climate, who would want an airport that essentially requires a complete rebuild to bring it up to date?