Yes, I had yummyfajita's question and this does make sense... a "niche" (this is pretty big to be calling "niche" really, but let me discuss the generic case) could afford to charge a bit more of interested customers and focus that stream on somewhat more pricy licenses.
Which, once again, just makes me think that Netflix is, if not "cutting itself off at the knees", at least hobbling itself with such a low flat-rate price. On the other hand it can hardly afford a complicated "full cable"-style offering, but some sort of middle ground may be something they're forced to do someday. Hulu, too, for that matter... even as a subscription service focused on TV it seems to me that the selection is limited by the fact the subscription price is just too low.
In the meantime, as a consumer I have no trouble mixing a nice Netflix subscription for flat-rate stuff and mixing in per-show fees from Amazon, but it's still money Netflix seems to be leaving on the table.
The marginal value of content with a strong, dedicated, but niche following to Netflix -- whose branding is mainstream and not targeted to that niche -- may be less than it is to a provider specialized in targeting that niche, which means that the licensing fee Netflix could afford to pay is less.
Further, your argument presupposes that all streaming licenses are non-exclusive. That is, rather decidedly, not the case. If someone else secures an exclusive license, Netflix cannot license the same content and provide customers with a superset of what the niche provider offers.
> It's also unclear to me that Netflix is even lacking in this specific niche.
It probably isn't compared to any major provider that a consumer is going to find today. OTOH, it could be in the future, especially if a more targeted competitor exists that starts outbidding Netflix for exclusive licenses for content in that niche.