From what i've seen FR more or less fill 2 flights a day to STN. A large percentage of the SLF are from the bordering counties of the Republic. It takes a good hour to drive from LDY to BFS so I don't think FR could expect these passengers to follow them to BFS.
If, as it seems, FR have good loads out of LDY then why walk away? BFS has a different catchment area with both easy & Go routes to LTN & STN. I'm sure they could compete and add more destinations, perhaps to Brussels as I understand Sabena are considering pulling out.