After doing a bit of investigating on ANN, I see Kupo's point. As far as their database is concerned, Goro has done no work on any anime at all, which is the best reason I can see that explains why Hayao is still against it. Of course it is perfectly possible that Goro has natural talent but because he has no past experience we just don't know.
Things were different in the case of Hiroyuki Morita, Ghibli's other new director: he worked on a number of other projects prior to being placed in charge of directing The Cat Returns, which included storyboards and key animation. Everyone has to start somewhere, but placing someone in charge of a film with no prior experience of the techniques is a bit risky. If Hayao is unhappy with Goro directing because of this, I'd agree with him.