I missed this the last time I read.
I think this really does get to the heart of things. It's all about genre expectations. Walls of Iron don't break the world because that would wreck the genre, so, we just don't do that. Sure, we can come up with various work arounds and reasons why not, but, at the heart of it, that's why.
And, as reasons go, it's not a bad one. Don't read the rules as the reality of the setting, and we're pretty much good to go.
I think you've nailed the crux of your original post's problem.
I learned this lesson in Battletech. You could find all sorts of explanations of why battlemechs couldn't work by players of the game. That way lies madness, as you are basically ruining your own suspension of disbelief.
Many of Celebrim's house rules that he's posted have a trait I like, where they account for keeping these concepts in check (like his reasons people don't run rampant with Raise Dead).
I wouldn't want to require a mile of house rules to counter these things, but the idea has merit.
One potential house rule concept would be the idea of "Equivalent Exchange" stolen from Full Metal Alchemist. With this concept, casting Wall of Iron would require as much material of equal value. Thus, the wall of iron isn't actually free.