It should be factored in when talking about '4% average income 9% average rent'. It is reasonable to expect that what ever rent you're receiving in 2009 will be significantly higher in 2019, whilst your repayments will be within the same range as they were circa 2009, or less if you have paid off a portion of the principal.
By the way, of the two I have, one is returning just under 7% (and hasn't had any rental increase for almost a year and my agent recommends one though I haven't taken that advise), the other 9%, and I have never raised its rent. It was 9% off the bat... they can be found.
