Di Maria has never played in the premier league before, and before last season he wouldnt have been worth anything like £60m.
Mata has been anonymous in probably 75% of games for us since he joined, I would say. We all know what he is capable of but at no point has he been able to deliver on a consistent basis for us - not even last season when he had his best form, was he playing at a level that merited a 'world class' status, for me.
Its fine to rate players potential, but if they do not deliver on it then there must come a point where the expectation is changed. Just because a player had a great season a few years ago, we shouldnt hold out for ever more that he will (or can) return to that level.
What it boils back down to is what (if any) place Fellaini deserves in the squad. As far as I am concerned, the level he has performed at this season is good enough for a top side, and this appears to be where we disagree. Our best run of form where we went unbeaten was with Fellaini in the side playing 90 minutes every game, we had the best results and performance and we were the leagues form team. Do better midfielders exist in world football? Of course. But if we are winning our games and playing well, surely that consistutes "good enough" (in the context of people saying that he is NOT "good enough")?