It's a shame as it's a lovely airport, but the harsh reality is that it's only by a quirk of fate that the central belt even supports two viable airports. If an airport had been built somewhere halfway between Glasgow and Edinburgh then that would have been the one that survived.
The rail link is a plus point, especially now there are more trains, but it's not an inherent advantage of the airport that represents a reason to develop the site. In particular, for example, if the Scottish government throws enough money into the pit at PIK, they could have instead funded a rail link to Glasgow Airport (I'm sure it could be done more cheaply than the last plan; there's a railway line within meters of the airport boundary, and places like Birmingham survive without the actual railway station being right up at the terminal building).