I remember a few years ago Jet 2 started NCL-AMS up against KL twice daily. It then quickly went to once daily, then they gave up altogether. I think it only lasted for a few months.
During that time the KL frequency was maintained with a modest drop in LF. I dont know how the company will react this time if it happens.
Dont forget a large proportion of KL passengers are flying onward from AMS.
Of course EZY have competed with AF to CDG for a few years now and it seems to have worked. But its ending at the end of summer schedules and they will have to do something with the based aircraft. So new routes are likely to be announced. We will find out soon where to.