*None of that is for the OP to worry about, or for you for that matter unless you're thinking of getting a job working for Mumsnet?
I've never seen posts like yours on any of the other threads asking for specific topics.*
No I'm not thinking of working for MN or ever have done.
As for "none of this is for the OP to worry about" - you're right. I don't dispute that.
The OP (or anyone else) can request as many topics/subs as they like irrespective of financial considerations.
The reason I brought this up was to offer up (based on industry experience) why specifically the request for multiple subs that replicate main site topics and specific moderation are imho likely not to be granted (not to say they they won't and it's for MNHQ to decide) plus in the spirit of wanting such a space within the site to succeed explaining why a single topic to start with is likely to be more effective and gain more traction.
Finally to answer your final point about I've "never seen posts like yours" in respect to requests for new topics, I refer back to the point that I've been consistent in supporting this the creation of this topic but have never before seen a request that is asking not only for a topic but a whole umbrella section that replicates multiple site level subs.
Had I done so, whatever the subject matter of the topic, I would have offered the same opinion.