It's an interesting one. On the one side there's some truth in that the airports reflect the economic wealth of the areas they serve, but that isn't entirely the case. There is a valued need for public money to support crucial connectivity in some regions, like the NQY-LON PSO route for instance, but not an entire airport operation beyond anything short to medium term where it doesn't add significant economic benefit.
That's largely why an airport like Doncaster Sheffield reached the end of the road because it offered no critical, wider economic purpose to justify throwing public money at it when other nearby airports at EMA, LBA and MAN can easily serve that demand.
Cardiff I feel is a little different as it does serve some important hub links that are crucial economic drivers, and there is, I believe, a large enough propensity to fly among it's catchment area to support a sustainable long term future, returned to the private sector. The issue unfortunately is you have public ownership come in to protect the airport from closure, but you then don't have the expertise, nor the funds, of a private investor to drive that sustainable growth of the airport through competitive and meaningful growth incentives. It's never going to be a big airport, but unfortunately CWL is stuck between a rock and a hard place at the moment and that may only change once the wider economic picture picks up so that a private investor can unlock the potential that's there.