How does the little kingdom survive against the huge kingdom attacks (or even worse against multiple kingdoms attacks) if they don't have the option to move their treasures?
It does not and it does not survive now either. You destroy their treasuries, so it will not profit from the treasures either. You take the treasures anyway, hidden or not. But the small kingdom will have a chance to fight back. The big kingdom gets no VPs from the treasures, but the small one does, so it tilts in their favor, as they will have a realistic chance of stealing treasures and VPs from the big kingdom.