(Plymouth's) catchment area for football is the whole of Devon and Cornwall...
The simple geographical spread, or population size within a catchment area is not particularly important, unless you're talking about a compulsory activity like attending school. What counts is how easy it is to exploit the population of said catchment area and in the case of Devon & Cornwall that is very difficult to do. They are large, relatively sparsely populated counties with a high number of small, scattered settlements and poor transport links between them.
Most people are not going to schlep for an absolute age along relatively poor roads or crap bus and train services in order to attend even Argyle v Stoke in the Championship on a Tuesday evening. Penzance to Plymouth takes between 90 minutes to two hours and from the other direction it can take nearly THREE HOURS from Axminster by train - and don't even get me started on dicing with death in poor weather from North to South Devon.
Even travelling between Plymouth and Exeter is much more of a pain than it should be. I think the 40 miles took me about an hour and twenty minutes by train last time I went to the derby when I was staying in Exeter.
There is also the compounding factor of the kind of people in the catchment area. There is not a huge tradition of passionate interest in football among the local population and a high proportion of residents who are interested are incomers from other regions seeking the rural idyll. They tend to bring their allegiances with them.
You could put a minor-league baseball team in the wilds of Canada that would probably have a larger nominal catchment area than many of its rivals , but that wouldn't mean very much in any practical sense.
I have no shame in saying I come to watch the Exeter Chiefs play sometimes, in fact regularly go to the corporate setup with work. If we were ever to make it to the Prem I have no doubt people and businesses would travel from Exeter to come and see the football too.
Would rather money was spent sorting out the Plymouth - Exeter ridiculous rail line.
I think considering the above it's not unfair of our fanbase to want us to be a consistent Championship side. Think we've missed the boat though. Too much money floating around the top 2 leagues and the gap has become huge.
In a sane world you should be able to dream of all the stars aligning and somehow tasting the top flight, while we should be able to dream of spending more than one or two seasons in the Championship if we really lucked out with everything going perfectly.
IMO the only thing that can stop things getting worse is for the big clubs to eff off and form some kind of closed shop super-league, taking the incentive for smaller clubs to join a dangerous spending arms race with them.