But they already went the F2P route with Lost Swords. Why would they do it again but call it SoulCalibur 6? Lost Swords doesn't even seem like it's been particularly successful, either.
Lost Swords made truckloads of money for Namco. They are still making content for the game which speaks volumes for how successful it is. To put that into perspective, Namco abruptly stopped making content for Tekken Revolution which on its page has many things like Julia and Bob's premium effects are still "coming soon'. So they know when it's time to stop investing.