Why not use the train - it takes 1h15 including the shuttle bus to airport front door, and is hourly.. , the bus is almost never going to be faster than waiting for the next train. A taxi for the 12 miles from central Cardiff also won't cost much more than public transport if travelling in a group of 3-4.
So, nope, public transport is not the cause of Cardiff's woes. It has a large population nearby, all west Wales, Swansea and the Port Talbot/Neath/Bridgend/Newport etc catchment. Bristol is nabbing this with a weaker catchment area.
Plenty of other airport connections have this problem but seem to get a lot of traffic (eg. how many services an hour between Bimingham Airport and Banbury/Oxford/Reading.. - and I for one use it and there are plenty of others doing so!!)
If necessary it should be considered against all the other TFW priorities - maybe they could do some sort of "review of shape of future services" and publish it for consulation/feedback?
No. It's a good view. There's a fixed amount of money, unless you are bringing in new revenue - and without the extra money, you're taking it from another place.