wb9999 -
Where to start?
The whole point about airports such as SZD is that they are there to link with other regional airports (as well as hubs), not compete with them (BRS - LBA, ABZ - HUY etc.)
Second, what does it matter how long the runway is if there aren't enough people in the local catchment population to fill aeroplanes?
I assume you're aware of the Ciudad Real fiasco? 4,000m runway, and yet, surprise, surprise, no passengers. Result - opened October 2008, closed April 2012.
Obviously, DSA is not doing as badly as Ciudad Real, but it's clearly not making money either.
The sad fact is that DSA and SZD will both be catering to niche markets. Unfortunately for DSA, its niche markets do not come with big margins, and its costs are outweighing revenue in a big way. SZD on the other hand is much better-placed to attract high-margin traffic, and to make money by combining that traffic with other on-airport while keeping costs low thanks to smaller infrastructure.