A huge downside of that it that adding a new (public) struct field will be an incompatible change. And what people will do is add a non-public struct field with

A huge downside of that it that adding a new (public) struct field will be an incompatible change. And what people will do is add a non-public struct field with a setter method, both to avoid the compatibility break but also because for many of these fields the defaults are just fine (even though sometimes they're not) and no one lines having tons of boring boilerplate.