Do people really think they would release a F2P game without a purchasable addition with it right away?
Isn't that the whole point of making something F2P? You let them play the game for free, they get hooked, and then they want to spend money on more of it that wasn't free. Take Firefall for example: you start playing one of the frames for free, see cooler ones being used by people, and immediately want to throw down cash for another one.
So, what would you do?
I would release a game called "Hearthstone: Heroes of Azeroth" and make it free to play. I would also already have developed a booster deck called "Hearthstone: Warlords of Draenor" that is even cooler, with more unique cards, and then charge people for it. I would then continue this pattern for years and years making bank in the process.
A few months later you'll see something along the lines of "Hearthstone: Dreams of Emerald" followed by "Hearthstone: Flames of Xoroth," and so on.