No one here says it should be this way. But honestly since I moved to Taipei I have not seen a shred of evidence that the area would be developed otherwise.
Songshan Airport not a “novelty.” Tokyo has one of these things (Haneda), Seoul has one (Gimpo), and so does Shanghai. You could argue that these are vastly larger and more important cities than Taipei. But certainly by utilizing Songshan Airport in this way, Taipei is not embarking on some radical novel scheme.
London also has a City airport for domestic and short-hall flights.
Like others say, dont trust powers that be with the space. Songshan Cultural Park just seems like a few restaurants, a massive Eslite and a pond.