Even a current homeowner might want to move one day, and if house prices uniformly fell by 50% relative to current earnings, they would find themselves more able to upgrade their house. The only people it would hurt would be people moving from owned accommodation to rented accommodation or cheaper owned accommodation.
What am I misunderstanding here?
EDIT: It seems I was forgetting a key fact: mortgages. When people hold a mortgage and their property value goes down, they can be left holding the bag.