In a aroused state, it wouldn't matter what you're watching, it could be someone just running a hand over their other hand and you'll get viewers, it's just enough to get off on.
It's not "will the audience look for porn", it's "Why will they choose your porn over someone else's?"
(Likewise, it's not "Will the audience look for an action-adventure show", it's "Why should the audience choose YOUR action-adventure show?")
Most successful porn sites cater to a specific fetish, instead of a bucket of random porn.
He needs to find his target market. Not sure who it should be, though.
Point being: there may not be thousands of sites competing, but when the search returns over a million sites and its nearly straight femdom out to the thirteenth results page (and likely beyond)...I'm going to wager that you didn't do that very simple bit of research, a google, before making such a declaration.
As you yourself said, the actual jargon is "femdom porn", not "female domination porn", or even simply "femdom".
By adding "porn" to any keyword search, you will get many extraneous results (see "hacker news porn" above).
Most importantly, these "alot of results" all have affiliate banners to the same handful of "femdom" sites.
Next time, try looking at the actual content, not just the Google results. I should have taken you up on your wager!
I will cede that there are a few fantastically successful ones, like I suspect in most media, which are rather strongly linked to...