Ever since Natasha Kaplinsky left they've promoted the weekend presenter. Natasha > Sian > Susanna and now Louise. The same happened when Dermot Murnaghan left, Bill Turnbull was the weekend presenter then.
I think there's a real logic in the path Breakfast have taken. Arguably, the least successful presenters were those parachuted in (Natasha and Dermot). The fact Breakfast is a seven day operation has helped breed good presenters by using the weekends to try out presenters when the usual weekend presenter covers weekdays. Having said that, I hope they use this opportunity to sort out the rota a bit and somebody does 4 days and somebody else three. I like Naga and would like her to become the second stream presenter on a permanent basis.
Also I've always thought Louise and Bill had better chemistry than Susanna and Bill, so this could be a good thing