The only reason I think LTN may struggle is that FR would need to offer very low fares to tempt passengers to try the airport, ORK seems to be "used to" STN, LGW and LHR and between them the airports offer fairly good geographic cover for the London area.
The other big airline at LTN would be easyJet. I don't see them flying into ORK any time soon. (or any Irish airport, for that matter)