At some level you have an average revenue per user.
If you charge up front some reasonable multiplier of the net present value of that average revenue per user, what is the difference if it's spread out over 10 years or immediate?
I guess my point with that is that as a user, I see free to play in general as being somewhat duplicitous. If it has a price tag on it, I can buy it or not. Even DLC, bad as it may be, has that sort of thing going on. But 'free to play' really means 'I won't tell you how much this will cost you up front'.
The bottom line is that your game has to be really good to make money, because there are so many games out there and many people want to make games, going so far as to spend large amounts of their free time doing it.
Competing with 'free labor' is hard to do. There's no silver bullet, and no business model magic will change that.
It's the same as being a rock star musician, or a professional athlete.