Not everyone who flies is concerned with choosing the airport that is closest to the city centre. Prestwick is only 40 minutes from Glasgow city centre, which is roughly the same as Heathrow, Gatwick, Luton and Stansted are to London. I don't hear anyone complaining about those airports being too far from the city centre. Prestwick isn't exactly in the middle of nowhere as some would have you believe. Also not everyone who flies wants to fly from a gigantic shopping mall with endless corridors or buses or trains out to the satellite terminal.
As I have said before, I do not wish any of the three central belt airports to close and I hope that all three enjoy increased activity which in turn brings tourism, business and jobs.