This is the problem Barnstable - you don't actually understand what a population catchment is. There are two different population catchments (time and distance). It is the point on a map where the time travelled to get to your airport is equal to the time travelled to get to rival passenger carrying airports. Alternatively it is the point on a map where the distance from your airport is equal to the distance to rival passenger carrying airports. They are both finite areas of land and the population of people therein.
Yes you can induce people to travel from Plymouth to use DSA but the time and distance catchment populations are locked figures. By definition EACH airport has its OWN time and distance population catchment. DSA has a very small population catchment.