I don't think they made a mistake with KP, they need to have a London base. You can see King William continue to reside at KP in London, and use BP as the office. This will also allow BP to be open a lot more to the public.
I think they will carve our a comfortable private apartment at Windsor Castle, which will serve as their "main" home. I do not know if this will be a materially different space to the private apartment of QE2 or KC3 at Windsor.
I think they will give us Adelaide Cottage. It will have served its purpose of providing a "normal" home for the children in their formative years growing up.