I'm not sure BFS would be jumped onto by another airline should EZY pull out.
BE offers actually a very competitive and high frequency service to BHD that appeals to both BHX and BHD originating passengers.
From the BHX end, the first BE flight arrives in BHD by 8am, the first EZY however gets to BFS not till around 9:30am and isn't normally that much cheaper than BE. I'm surprised actually it lasted is long as it has.
Unless FR pick up the route and commit to at least 3 or 4x daily and can regularly offer £19.99 fares as an example, I'd struggle to see anybody else picking up this route.