When we say learning to write should be efficient we mean that, in an ideal world, all students should be able to learn how to express themselves across all main text types by following a known series of steps and investing a known amount of effort.
That's not to say everyone should be a professional writer.
That's not to say all writers should be as good as each other, or learn at the same speed, or feel the same way about writing, or write the same way about the same things.
It just means that we should be able to say to any student with basic literacy, "If you do these activities with moderate effort for 100 hours, we guarantee you'll know the essentials of the four major text types. And if you practice the same skills for another 100 hours, we know you'll be reasonably fluent."
If we can do that, then we get to live in a society where everyone knows how to write and we can all focus on content (and style, if we want to be craft nerds).