That may well happen, but for people who want to use Google Voice now, $3 is a pittance to have a better interface to use it. With the animosity that is growing daily between Google and Apple, it could be months before Google releases a native app. Or they could decide to keep it for Android only.
Skype works pretty good on my iPhone. The only thing I don't like about skype via iPhone is that skype puts a heavy drain on the battery.
Marc