We're assuming we all somehow have perfect customers with technical knowledge who know exactly what they want and can express it as such, while gracefully accepting pushback over constraints brought up.

Anyone who's worked in a "bikeshed sensitive" stack of programming knows how quickly things railroad off when such customers get direct access to an engineer. Think being a fullstack dev but you constantly get requests over button colors while you're trying to get the database setup.

Isn't it a bit of both? When it comes to noticing whether or not code will be a security nightmare, a performance nightmare, an architectural nightmare, etc, haven't experienced developers already learned to watch out for these issues?

Too right. Drilling into the domain from first principles and with critical faculties enabled unlocks so much more value, because the engineer can then see much better ways to solve problems.