Peba, I understand what you say about Ryanair maybe having to change their business model but that's a bit in the future. In the short term if Infratil close Prestwick if there is no buyer for it as a going concerm then MOL will have no option but to close his PIK base. End of.
As for freighters continuing to use the runway and cargo facilities in the event that only the passenger terminal closed, I doubt that there would be a sufficient revenue stream from them to pay for airfield upkeep and even limited fire cover etc. I suspect the cargo carriers would simply decamp, possibly to Robin Hood which isnt too far away and has absolutely first class facilities.
A sad state of affairs