The fact that landlording is bad and not a profession isn't the point.
The point is that @MithranArkanere@lemmy.world's argument failed to convincingly argue that because it was logically fallacious:
https://en.wikipedia.org/wiki/Fallacy_of_division
In other words, the fact that thing A would "destroy of the economy if everyone did it" is an emergent property of everyone doing it, which doesn't apply to any single entity doing thing A.