I agree with Mark and Lauren.
Here are some reasons I think justify leaving Beta without a large volume of questions:
The coding and OS generate more questions because programmers (and network administrators and other techs) have very specific tiny detailed questions, bugs, and/or error messages to address, and they are indeed slightly different by the thousands, not duplicates that can get the same answer.
IMO this is just not true of "writing". In "Writing" this would be the equivalent of critique, plotting, or What to Write. Many Stack Exchange coding answers will both critique their OP's code and provide What to Write, cut-and-paste syntactically correct chunks of code that do the work requested. If the OP asks "where is my bug in this code" the answer is usually "write it like this". I know several programmers that have used dozens of these code snippets in their projects.
The rules of what is on topic here naturally restrict the questions that can be asked and reduce the volume of questions. I also think that guidance in writing is just generally broader; a site that did not provide specific answers to specific problems in coding, and only provided guidance in good coding style, would probably have even fewer questions than Writing!
I also think the more generalized type of Writing questions we answer will necessarily have a larger footprint than the specific questions about coding or grammar, and this broader footprint leads to more duplication and overlap of questions, and perhaps fewer questions if potential queries are short-stopped by a basic search.
Worldbuilding, for example, has a lot of questions because is like coding, the problems are specific. Map critiques and logic critiques (eg. on the rules of magic) are welcome, the equivalent of "What to Write" is not prohibited; e.g. "I need a flat plain in which a forest borders a desert, what natural phenomenon can cause that?" Or, "How can I make spiders go extinct?"
I am not advocating a change of on-topic rules in this post, just saying there is reason to accept we are a mature site without meeting some magic threshold of question rates. I should think that, like writing itself, the rate of production of interesting material is much less important than the number of fans!