@pug
Well, I'd use pax numbers as a good first approximation of viability - the data is available, as s7s has kindly shown. If I wanted to get a tad more sophisticated, I would look to look at the total and high income population in the airport's catchment area - I might try to get some data on house prices as a proxy for the latter. I would also look at time/distance to next nearest airport and identify island airports. I would also include freight usage as an additional source of airport revenue. Put all into an econometric model, place in a warm computer and cook the figures until ready. Yum.
On regional planning, let's look at the potential value added of intervention compared with the market solution. How might markets not give a socially optimum solution? If there are two marginal airports and only enough business for one: both will close unless they either come into the same ownership or there is intervention. If the airport land is temporarily more valuable as housing but closure would have a impact on the regional economy that cannot easily be monetised. if our friend the megagarch decides she wants airport A to stay open, even though it is in everyone else's interest to keep airport B. (Of course, in this case, she might flounce off and buy Italy or something).
So, there is a potential case for regional planning, if reasonable decisions can be expected. This may or may not be the case.