Considering 70% of houses are owner occupied, and 30% are tenanted (or thereabouts) ... who is to say that anyone is buying for yield at any given point?
Not every purchase is about yield ... in fact only 30% of purchase are for such.
So, 70% of the time people are buying ppor's - and as the top half get wealthier they are spending more on these properties.
You would also probably find that the smart investors in this wealth category are not "just" buying ip's, but rather creating them by multi unit developments etc - which, in turn, increases their yield as they are "buying" at cost price rather than retail price.
As for vacant rooms? There are three of us in our current ppor. It has 5 bedrooms (there used to be 6 but the older kids have all moved out). Technically we need only 2 bedrooms - but because we can afford the house without renting any of the 3 others out, they ain't going to be rented out in my lifetime here.
One is now an office, one a playroom and one a spare room for when the rele's regularly visit.