In most cases, a home isn't going to be rented forever. At some point properties get liquidated by the owner. If it's an individual, most likely at retirement or when they don't feel like managing the properties anymore. For a company, it's when it no longer profitable or when the company goes under.

My point is that properties, even rental houses, turn over at some point and there is typically enough turnover that it's probably not going to tie up affect the long term percentage of homes for sale.