If your question was whether there are some unethical landlords, the answer would surely be yes. But you asked if renting living space is a "fundamentally unethical practice." Your implicit argument that it might be is that "at this point" (at which point?) a landlord puts at risk the most inelastic needs of human beings, placing them behind more or less arbitrary paywalls."
Let's agree: people need shelter. They also need food. And clothing. And in very many cases, transportation. And medical care. And many other things. And let's agree, at least for present purposes, that a society that doesn't have a reasonable way of providing such things isn't doing what it should. We can even put it more strongly: insofar as we can talk about obligations that a society has, let's agree, at least for present purposes, that societies are obliged to devise reasonable ways for providing these things. The word "reasonable" is covering a lot of territory, but I don't think that will affect the point I'd like to suggest: what if it turns out that allowing people to be landlords , if properly regulated, is part of a reasonable scheme for getting people housed?
You say that landlords put up "more-or-less arbitrary paywalls." The landlord would reply that s/he has incurred up-front costs and ongoing responsibilities when s/he becomes a landlord. We could agree: if landlords create a situation that makes it difficult or impossible for people to get affordable housing, governments are obliged to take steps to ameliorate the situation. (What steps? That would be another, much more complicated discussion. We're trying to get the background issues straight.) But it doesn't follow that allowing people to rent living space is allowing something fundamentally unethical.
We could work through various kinds of cases and various scenarios. I have no doubt that we could come up with lots of scenarios—including real-life ones— in which landlords are acting unethically. But as someone who's rented living space in various situations over the years, I also have no doubt that we could come up with lots of scenarios in which it would be pretty unobvious that the landlord is doing something "fundamentally unethical." I've had landlords who've provided safe, well-maintained living space at prices that didn't impoverish me, even at times when my income was not high. The arrangement worked to my benefit and to theirs. I'm surely not the only person who can say this.
From this it doesn't follow that there are no stereotypical slum landlords. There are. And nothing about the overall health of the rental market follows from what's been said. That's an empirical matter on which I have no expertise. But to show that the very existence of landlording is immoral would take a lot more premises than are on offer here. I won't say that I'm sure the case couldn't be made. I'll just say that I have my doubts.