There are ways in which it can be done as it has been done in the past. Don't foget that Bristol was operating at similar levels compared to Cardiff if not lower for quite some time. However the failure to win the bid for a GO base was a start to the problems.
That's only part of the picture.
The gap between the two airports actually began to widen after 1986 (the last year in which CWL carried more passengers than BRS - 487K against 469K: both airports were largely charter fields then and CWL did have a slight edge most of the time in the 70s and early 80s). By 2000 BRS was handling 2.124 mppa and CWL 1.500 mppa (100K more than in 2010) and the only low-cost airline around Severnside then was Ryanair which operated only to Dublin from both CWL and BRS. All stats are courtesy of the CAA.
It's absolutely true that the major growth at Bristol occurred after Go set up its base there in 2001 (their second one after Stansted) and, according to Barbara Cassani's book, the short list after an eight-month search around Europe came down to Bristol, Newcastle, East Midlands and Glasgow/Edinburgh. She doesn't mention CWL so we don't know if they were in with any sort of chance.
The key point for Go, if it had come to a straight shoot-out between CWL and BRS, was that, 'The catchment (BRS's) was along the wealthy and populous Thames valley corridor and the West Country was full of well-off older people with time to spare.' (Cassani again).
Not only is the core BRS catchment larger than CWL's but the make-up, both business and leisure, is broadly more prosperous. It has to be asked why airlines and passengers have in increasing numbers taken to using a small airfield on top of an often misty/foggy hill, with a constricted runway and which airline pilots have told us on PPRuNE on many occasions can be extremely challenging under certain weather conditions when there is an airport across the river that has few of these difficulties. The answer can only be one thing - the market.
The CWL master plan has for several years pointed out that it must retrieve those passengers that use neighbouring airports such as BRS and BHX, but it must do more. It must attract passengers from outside its core catchment and its target areas are the West Country and the South Midlands. To do this it will be necessary to take passengers from the core catchments of BRS and BHX.
Given that only four years ago CWL was handing nearly 2.1 mppa there is obviously scope to get back up to at least that level though if a roll takes it that far it's not inconceivable that growth could continue, but the 5 mppa-plus mentioned in the CWL master plan might be a long time in coming.
There is an arguable debate that BRS has overperformed, especially having regard to the location and make-up of its airport (CWL might be in an even worse predicament if the Bristol Corporation had elected for Filton instead of Lulsgate when they moved their airport from Whitchurch in the 1950s).
Where BRS wins and CWL loses is in the ability of the former to more easily attract passengers from outside its own core catchment area, and the fact that on thinner routes BRS needs fewer passengers to 'top-up' flights from further afield than does CWL.
As a regional airport CWL is doubly unlucky in having a near-neighbouring regional airport serving such a prosperous area and, if that is not enough, another even bigger one (BHX) not a million miles away.
There is no doubt in my mind though that CWL ought to be doing better than it is.