I don't think Koumetio is behind VdB. We don't seem to rate him very much at all, and he seems to have been overtaken by everyone.
It's hard to say where he ranks relative to Williams and Philips. Given that he played in the league cup, I suspect he might have been ahead of at least Philips. But now Philips has done well against West Ham with him out injured, it'll take a bit longer for Billy to break through. Just a matter of luck.
There's also a matter of maturity which Philips brings as someone that's played that little bit longer. With so many injuries, it may be that we value that characteristic a little bit more than we would if Billy had a chance to play next VvD, say.