My understanding is that FR have a 'Nil' landing charges agreement with BRS, which is why O'Leary moved there when CWL upped their charges on the DUB route back in 2006.
BRS struck the deal with FR as they could make up for it with ancillary spending within the terminal. Something which CWL doenst have the capability to do so due to the size of our departures area etc, but im sure that CWL could make it work if the need arose at anytime.
With regard to your Delta, EK theory, then i do agree, but as TA states, this would cause uproar across the severn. Back before the ryder cup, delta had gone as far as negotiating and securing landing and departure slots at CWL, so they must have been told by WG that the support was available, as they wouldnt have bothered without it. But then something must have happened as Delta pulled out of negotiations, for reasons which i nor others seem to know.
The way forward for CWL is to attract more carriers similar to Vueling such as Air Berlin or its low cost subsidiary, Niki. Then, when PAX numbers are back upto respectable levels, i.e. approaching 2 million, then the airlines that you mention may take note of CWL. The hardest struggle is getting upto around the 1.6 million mark again, which needs to gain around 500-600 thousand passengers based on current levels. Lets just keep supporting our existing routes and trying to exploit areas that BRS doesnt have covered !
Also the SFB charter with MON begins a week tomorrow, so that will help support passenger pevels a little more !