I'm not sure Minecraft counts, because it started out as what we might call an early access game today. That is, it was very, very incomplete when the first purchasable version was released, and pretty much exploded very quickly after it matured. The numbers were certainly impressive for an indie game released only on PC at a time when PC gaming was, in a way, at a low point. All things considered, I think it grew really quickly.
I also don't think the remark about Pokémon Go is appropriate in this context, as Pokémon was already huge before it. The way the paragraph about Pokémon is worded makes it sound like Pokémon Go is the main reason to the brand's popularity.
I can't think of any sleeper hits. I'm pretty sure I've though of one years ago, but I just can't remember it, and no others come to mind right now. Sorry!







