Obviously he's droppable, everyone is. But people are making predictions on the only evidence we've got, which is what happened last season. Based on that, Herrera is clearly the strongest candidate for the point of the midfield triangle. And that's why everyone's putting him there in their line-ups. It's not 'Herrera love', it's common sense.
I'm not sure how you can 'see' Rooney playing there, given that he's our only top quality senior striker. Unless/until we buy someone else, we have to assume he's our regular #9. Or Schneiderlin, given that Van Gaal literally just said that he, Carrick, Schweini and Blind are competing for the deeper midfield positions.
Mata at least makes some sense as a suggestion, and he was good at the end of last season. But Herrera was more important, and Herrera was actually in that position, so it's logical to assume he'll stay there.