You don’t need complex sentences to express complex ideas. When specialists in some abstruse topic talk to one another about ideas in their field, they don’t use sentences any more complex than they do when talking about what to have for lunch. They use different words, certainly. But even those they use no more than necessary. And in my experience, the harder the subject, the more informally experts speak. Partly, I think, because they have less to prove, and partly because the harder the ideas you’re talking about, the less you can afford to let language get in the way.
Informal language is the athletic clothing of ideas.
I’m not saying spoken language always works best. Poetry is as much music as text, so you can say things you wouldn’t say in conversation. And there are a handful of writers who can get away with using fancy language in prose. And then of course there are cases where writers don’t want to make it easy to understand what they’re saying—in corporate announcements of bad news, for example, or at the more bogus end of the humanities. But for nearly everyone else, spoken language is better.