Again, I think you're mistaking what people are saying.
> Pretending that, just owning property and having them gall to dare to not rent it at or below cost is what's immoral or unethical is harmful to persuading anyone currently undecided[...]
This is not what people are critiquing. No one else is talking about the thing you are. They are expressing their own moral judgements about a certain category of economic activity. That's why I linked you to a definition of "rent seeking." Economics does not make a moral judgement about rent seeking - it just says systems of exchange are more efficient w/o it.
Not everyone agrees on where to draw the line between productive business and rent seeking - but when someone speaks about businesses or owners "seeking rents" they are specifically talking about a proportion of their business that is past the point of making things worthwhile for them and into the range of excess. The definition of a "rent" in this context is that the business owner would make a reasonable profit without it. If they need to charge as much as they do to break even, it is not a rent (with some exceptions[1]).
There is another conversation all together that happens outside economics about how we should organize our society. The 'left' side of that conversation often includes the ideas that all landlords are bad or that profiting from property you own w/o labor is bad. Those are all ideas we could also talk about but they are orthogonal to the economic discussion. Economics cannot tell you what system of ownership is moral and morality cannot tell you what economic systems are stable.
[1] If an owner makes a bad investment and, say, buys a building at the peak of valuation, and then tries to pass on their bad investment to their tenants an an unethical way, that could be considered a "rent" in the economic sense as they are tying to get income on an unfair value of the building.