Perhaps MT doesn't want to move out of Berg Castle?
I am not sure if MT ever spent much time in Berg Castle. It is the official residence and very big and maybe not too comfortable. I think she spent most of her time in her apartment in Paris and Biarritz.
As the hair Gui has to take residence in Berg Castle and I have read that they want to build an annex to live there. There must be a reason for that. When retiring Henri and MT should live at Fischbach castle which is much nicer (at least looking at it from the outside) but no announcement was made.
Therefore I guess they will spend their time in Paris, Biarritz and maybe Casaubon once they have retired.
Spending lots of money to build an annex where there is already so much space to live in, is a bit strange or a waste of money.
However the money comes from their private funds and they can do with their money whatever they want to.