Novelist: Any author who thinks a sentence is ungrammatical will not write it down in the first place, and thus will have no need to use a grammar book.
Conclusion
Grammar books are useless as reference sources for authors
Evidence
thinks ungrammatical → write → consult sure grammatical → consult
Evaluation
"Useless" is so strong. All we need to push back is to argue that .
However, these two conditionals try to convince us it will always be useless. In both cases, the author won't consult the grammar book. Is there a way to argue that a grammar book is useful, even though you won't consult it? Maybe it's a paperweight or a weapon? Oh, wait. We have to be arguing that it's useful as a reference source.
Goal
If we weren't able to crack the riddle, then we would just go to the answers now thinking, If most people want it, we won't have enough. If too few people want it, we'll lose money. Thus, we're screwed" would leave out the in-between possibility of "what if the # of people who want it is less than most but more than too few?")