If this gets up, landlords will simply have to charge tenants an economic rent for existing rental property. Tenants will have to cover the actual cost of owning/running the property. Any net loss will no longer be subsidised by the Gov't and landlord, 30% and 70%, respectively, assuming a marginal tax rate of 30%. Why would any investor buy/keep existing rental property and rent it out at a huge loss?
New property will always carry higher rents and rents will simply increase across the sector.