I think that more important than the physical skill of writing complete sentences, is the MENTAL skill of CREATING complete and interesting sentences. Even if that means they just narrate them and you write them down.
It's just a biological and physiological fact that when kids are young, most of them will have their physical writing skills lagging behind their mental creative skills. This is NOT due to a lack of 'training' or practice, it's purely developmental. Making them write everything actually hobbles them... the slow process of beginner writing limits the amount they're able to write, limiting the extent of their creativity they can express, if you see what I mean. But if you let them TELL a story, they can come up with amazing, detailed, and lengthy stories.
The technical details of form and structure can easily come later. They can be taught and learned at just about any age. But if the spark of creativity is trained out of you, it's much harder to 'teach' that later.
Of course there are kids who are just naturally more inclined to write and be skilled at those technical elements, and it could very well be that SWB's children were in that category and she's extrapolated that to mean that if ANY parent does what she did then they'll get the same results with their kids. I think my daughter might end up like that. She's almost 5, she's already doing level B Math -- not because I'm making her out of some sense of needing to be ahead or train her or anything, but because she loves it and ASKS to do it. And she's just started doing a lot of self-initiated writing. It's all the 'invented spelling' but I'm encouraging it because she's KEEN and wanting to exercise her creative, and she's really trying hard to 'sound out' her words when she writes them. We're planning to do mostly Charlotte Mason style with her, which has lots of narration and also some dictation.
My son was the opposite when he was young - he didn't like to write, colour, do math, do any work at all. I tried to force it for the first few years before I learned better. A few years of unschooling and very light, creative schooling helped to undo the damage I had done!
We then let him DRAW his 'written' responses and that unlocked everything! I decided -- and realized -- that the mechanical process of writing well is a separate skill than understanding physics, or history, or whatever. You have to be able to write an essay to do well in college, but you don't have to be able to write an essay about a particular subject in order to prove you understand that subject. There are many different ways to express that knowledge. And so we did gradually keep working on his handwriting, but in no great rush, and separate from his other work. Yes, his sentences were mostly fragments, but that's separate from the knowledge. I think that's Bauer's error -- the idea that not being able to write well means you can't learn other stuff!
This past year, when he turned 13, he finally started showing *ease* with his writing. Still not great long paragraphs, but no longer complaining about it. So now we're finally getting into the mechanics like paragraph structure, good form, etc, and by next year will be well into essays. He's WILLING to do it now, when it would have been a fight before -- which would have turned him completely OFF the idea of writing. Now he's willing and even ENJOYING it, so it's setting him up for a lifetime of enjoying writing and doing well with his essays. Forcing it earlier would have made him hate it. And we would have been no further ahead *in the long run*. By the time he's 18 I guarantee you won't be able to tell the difference.