Harry Wayfarers,
I too use KLM a lot to get to the US and i honestly don't think they will pull out or cut their frequencies because of Qatar. Their offerings at CWL are much more broader. People use them to fly to China, the USA, Canada, South America, Africa and all over the world. Also a lot people use them to fly to places in Europe from regional France, Eastern Europe, Italy , Germany and Norway are examples of destinations that i've heard people flying to using them. Qatar's focus is on attracting people flying to India, the Middle East, SE Asia and China using Heathrow to and from Wales and the South West and of course to attract the 90,000 people a year travelling between Wales and Australasia. Many of the adverts for Qatar for CWL feature Sydney quite prominently.
As for Vueling i agree if a competitor turned up and dumped 10 weekly flights on each of their routes then they'd likely run but if someone like Ryanair added 2 to 3 weekly flights on their routes then i don't see them going anywhere, the example of that is Faro Ryanair added 2 weekly flights and instead of cutting or pulling off Flybe have actually expanded from 4 weekly to 5 weekly flights. Vueling have slowly expanded their frequencies at CWL over the years. Whether they will continue to or actually can add more frequencies only time will tell.
Why does CWL need an expansion of services? Demand for flights from it's catchment area is high and will continue to grow as Wales gets wealthier and as tourism grows. If you look at the Flybe base it is generally doing quite well. They've provided the flights and people are using them. Roughly over 4 million people fly to and from the airports catchment area every year. 1.6 million of those use BRS, 1.3 million use CWL, 1 million use London airports and the rest use airports like BHX and MAN. The demand is there it's just not being served from South Wales at the moment and that is part of the airports challenge to get as many people flying for the holidays, weekend aways and business trips directly from Wales and not England, to boost the Welsh economy with better business links and provide tourists from all over the world direct or better access to Wales so they visit our beautiful country and spend their money here boosting the Welsh economy. To do that it needs airlines and routes and more frequencies on routes as well.
P.S the numbers I gave are from 2016 and from a progress report/presentation done by the Airport to the Senedd at the beginning of the year. Pretty sure it's online somewhere.