Blizzard Entertainment to feel that Hearthstone was right to adopt its free-to-play model? According to a recent interview at Eurogamer, the choice was validated when the game was still in beta. When streamers were reaching the top tier of play without having spent any money on the game, it reinforced the idea that players could do well without having to spend a great deal of money.
Production designer Jason Cheyes mentions that there was always a concern that players would hit some sort of monetization wall, an impediment that the design team wanted to avoid. As it stands, while players can spend money to advance faster, winning matches and completing quests alone can serve to get gamers to the top -- and that's exactly what the design team wanted. It just so happens it saw it happening even during beta testing.