It works the other way... you can only have a repayment thats 80% of the market rate of the property.
Or do you think a landlord should be able to increase the rent so they can repay their mortgage faster?
Rental market does need more oversight. But their are bad landlords, bad tenants and bad legislation (such as needing to be actually evicted to count as homeless for access to social housing). Good landlords need to be encouraged.