I doubt many landlords are making that kind of money every year on a property and if they are you have to factor in their time and effort and the capital invested. These days with all of the changes many landlords are making small gains, some are making a loss especially as the gov now taxes on turnover not profit! It only takes one bad tenant doing lots of damage or staying for months without paying rent to wipe out years of gains.
Hum ......... any business needs a financial plan - which should include ALL costs and an assumption that revenue isn't 12 months per year. Not anticipating costs and assuming 12 months of rent AND having low margins is a recipe for financial loss IMHO