David, Prestwick is around 15 minutes drive while Glasgow is about 35 minutes, so there isn't that much difference. I've used Glasgow several times this year but the worst airports I've used have to be Heathrow (Terminal 5) in transit to Istanbul, and also Gatwick. After having had to queue to get on a train at Heathrow Terminal 5C to get to 5A, and having to get a bus from the plane to the terminal at Gatwick only for everyone to be told to wait at the wrong luggage carousel, catch the monorail to the South Terminal, queue to get on a lift to get to the train station and walk what seemed like miles, well maybe you'll understand why I'd rather use airports like Prestwick, Beauvais or Charleroi. At least at airports like Prestwick I can be in and out quickly. I know Prestwick Airport needs to be updated and there's not much in the way of shopping there but when I travel I'm not particularly interested in shops selling very expensive, over priced clothes, jewellry etc.
I know traffic at Prestwick has reduced considerably but I know airports that only have two passenger flights a day and seem to do ok. I'm sure the airport will survive and hopefully flourish under new ownership. To shut the airport down would mean unemployment for many people and that is something I wouldn't wish on anyone, having been unemployed myself in the 80s.