I think the biggest advantage of KLM entering the Belfast market, is the ability to connect to a range of long-haul routes easily at AMS. NI as a whole is very limited with full service carriers. Connecting to long haul really only being through LHR and EWR. BE I guess have a limited service to connect onwards at MAN and BHX.
Opening up KLM's route network from NI would be a huge advantage, and could see LHR numbers affected moreso than EZY AMS. I think back in the early days of EZY AMS, the flight was operated morning and evening for many seasons, I think up until CDG was launched, however could be mistaken.