@Iyapo1 and I were also specifically talking about spending diamonds on getting extra pieces, or an entire extra set, not on completing the first set.I don't really care about past events. A Free To Play Game Should Be Free To Play, for all players. If you want extras then sure, get your wallet out.
And while people keep saying that in practically every free-to-play game, optional things like the events are never included in that model. Only the core play of advancing through the story of the game is ever what is free-to-play.