The fact that there are potential landlords as re-purchasers in the future drives up the cost of new homes today, even if landlords aren't buying them new. It is very simple. If no landlords are allowed to buy the older homes then prices on older homes will be lower, which will also depress pricing on news homes, which will stifle development. You have to think about the secondary affects and not just immediate. Sure immediately housing would become more affordable, but long term development will decline and you will quickly end up with a housing shortage.

The moral of the story is really that the invisible hand is far better at efficiently allocating the resources of a society than any do-gooding central planner. Government intervention is almost always the wrong answer.