I'm not sure. I think there'll be more producers using utau in the future, like Police Picadilly with Yamine Renri. I don't think utau will necessarily outshine Vocaloid, what with marketing and popular figures like the Hatsune Miku and the other cryptonloids. Vocaloid gets constant updates,(V3,V4, etc.) But utau hasn't from Aya/Ame in a while.
I remember reading a thread about the future of utau or something, and that if the program isn't being updated, in time it'll end up being obsolete or unusable, but idk if that'll happen. Some popular utau might end up like the ones that used to have their long-standing fame, (Momo Momone, etc.) and so I think that utau's still long ways from surpassing vocaloid, giving those various factors.