Martin - totally bogus maths. Here is a proper analysis:
Take one long-standing tenant who otherwise would not be moving. His home is not available to let to a person on the waiting list, and is unlikely to be available until the tenant (and any other family members living with him to whom the tenancy might pass) dies.
Net availability - ZERO.
Sell the property and replace his home with a new social home. One existing social tenant is converted to home ownership. He is removed from the pool of demand for social housing, so the result is:-
Net demand = MINUS ONE, net availability = PLUS ONE.
Result - the household on the waiting list is provided with a new home that otherwise would not be available.