I’ve been thinking about this a lot since I watched it: even before the baby, Mike & Alison were struggling to do justice to the house and restore it or make it a viable business.
By selling Button House to the hoteliers, they ensured that it was fully renovated and preserved, and it had a future, whereas if they’d stayed it would have fallen into greater disrepair and they’d eventually have had to move out anyway.
So the ghosts, Mike & Alison & baby, and the house all got to move onto a happy future.