I'm talking solely about the marketability of a medieval fantasy RPG.
And the counter-argument to that is that is despite it being a medieval fantasy RPG, Skyrim didn't sell primarily
because it was a medieval fantasy RPG. It sold because it was open-world, immersive, and on PC had extensive modding legs. It also probably initially sold because it was an Elder Scrolls game, not something any other random medieval fantasy RPG game can automatically become.
It's the same way Witcher 3 isn't exactly selling because it's a medieval fantasy setting. It's selling because it's Witcher (which just so happens to be based on a book based on a fantasy setting)
Dragon's Dogma doesn't have the rabid, loyal fanbase it does just because it's a medieval fantasy RPG.
Kingdom of Amalur: Reckoning didn't sell just because it's a medieval fantasy RPG. (and witty ones might argue it didn't sell at all oh ho ho).
Heck, they're all good games in their own right but all added up to medieval fantasy RPG fatigue, just as there was WWII shooter fatigue, modern counter-terrorism shooter fatigue, and right now future war shooter fatigue.
If FFXVI is fantasy, you can bet it's going to be marketed on the shoulders of previous entries like FFXIV, FFXII, FFIX, FFIV, etc and not just because Skyrim sold well and is a medieval fantasy setting.