I don't think it is this simple. I don't think the ethics are this simple.

One case I can think of is property under rent control. If said property is sold for a price where the cost of any mortgage, taxes, maintenance, etc., is more than the cumulative cost of existing rents, is it "ethical" that the new owner should lose money? Does it make a difference if the new owner is a big faceless corporation or a family trying to invest for retirement?

What happens when there's been a good long-term tenant-landlord relationship, where the landlord has kept rent at just enough to cover costs, plus a little bit more, and then something throws that all out of whack? The municipality passes an ordinance that whacks the property with some substantially increased cost? I disaster causes damage that doesn't make the property unlivable but will cost significantly beyond what insurance will pay to repair?

I would say that a landlord, or potential landlord, has an ethical duty to help insure any tenants continue to be housed, but I'm not sure we should mandate even that. It because a slippery slope telling property owners what they can and cannot do with their property. We already have many limits that exist now, from zoning to ordinances to laws. What is the right balance?

What sucks is that I want to live in a world where all landlords act honorably. And while I generally loathe both sides-ism, and am very much on the side of tenants, I also realize that many, if not most, landlords aren't the soul-sucking corporate overlords that make such good press, but are individuals and small, local businesses with not a lot of margin--and who end up losing money they cannot afford because of ordinances and laws with good intentions that have unintended consequences.

Unfortunately, good law is nuanced. And nuance not only doesn't get press, it gets slammed in the next election cycle. Which is why we suffer the bludgeon of these laws. That all said, someone being forced to move isn't the end of the world. Sure, it's a PITA, and is especially tough on the poorest of us, but it's part of life that sometimes stuff happens and we have to do things that we would prefer not to, and sometimes moving is just that. And I'm not sure a blanket prohibition on that serves ethics or the public good.