For those interested in the technical aspects of the airfield, the NATS data for DND is available here;
NATS | AIS - Home
Runway 09/27 is 1400m x 30m with PCN 27/F/D/Y/T so that restricts the type of aircraft that can use the airfield and, more importantly from a commercial perspective, the payload range of such aircraft. As has been pointed out, it's difficult to offer competitive fares and make a profit using the kinds of aircraft that have been operated from Dundee in recent years especially given the high level of airport charges and APD that are imposed.
It might be possible to extend the runway to around 2,000m by buying the adjacent university playing fields and perhaps the runway could be strengthened to better suit A320 / B738 sized aircraft but who would be prepared to fund such an expensive project when the residents of Dundee and the local area have not made sufficient use of the services that have been provided in recent years?
When Edinburgh Gateway rail station is built this will further undermine the economic case for investment in Dundee Airport.
It's difficult to see a future for commercial air services from Dundee Airport without significant government policy intervention and subsidy. Is it best use of tax payer's money to be continually subsidising an airport such as Dundee given its proximity to Edinburgh, Glasgow and Aberdeen airports? Passengers from Dundee's potential catchment area appear to be voting with their feet and their wallets and choosing airports other than their own. If they want to keep their commercial air service, and if it's not already too late, they need to use it or they may lose it.