They would be able to offer more choice!
I doubt AF / KLM are really that bothered about offering choice.
If, as has been said, the HUY-AMS route is popular, then it makes no economic sense to compromise the performance of that route by competing against themselves. AF / KLM would probably rather have one route with excellent passenger loadings on which they could charge high yielding fares, than two routes of borderline viability.