If that property is sold and has 10 rent allowance and HAP tenants they WILL NOT get alternative accommodation and they WILL be made homeless.
Unless it was sold to a family of ten who were beforehand housed in emergency accommodation.
Again, the question was about the housing stock - its clear that the landlord exiting the market does not affect the housing stock.
But accepting your point about HAP tenants etc, then the solution is more housing held by non-property speculating landlords. An example might be that a buy-to-let mortgage is only attainable if the property is available to let for the full-term of the mortgage or even longer.