I'd like to see an example of a good writer, or even one sufficiently long piece of clear writing, that doesn't use idioms. Language is saturated with them.

You'd be on more solid ground (and closer to your point about loops) to say that, when teaching foreign languages to beginners, idioms are best avoided because the student is struggling just to know the individual words. Even then, though, I doubt I ever took an introductory language class that didn't teach at least some idioms; you can't get a feel for a language (or fully understand any real-world text) without them.