I've seen a lot of discussion along the lines of "These rules won't mean more houses come onto the market for sale - people will just leave them empty or put relatives inside them!"
But when my grandmother's house came to us a couple of years ago, we (my family) literally made the decision to sell it rather than try renting it out, because my father knew the way the wind was blowing, and had this long talk with us (me and my sister) about how the picture is gradually shifting in favor of tenants and this will make renting out properties very stressful and just not worth it in terms of time, money and energy.
So we sold it, and someone else owns it now.
I would be really surprised if the new rules did not at least increase the number of properties for sale to an extent. Yes, there will be some cases where people will leave properties empty rather than sell or lease them, but a lot of people will choose to sell and keep their lives simple.