Minecraft just announced its first new dimension in 15 years. Kids born the week the last one shipped will be starting high school before this one arrives. That gap is the whole story. Fortnite blows up its map every few months. Roblox pushes updates weekly. Minecraft became the best-selling game in history, 350 million copies and 212 million monthly players, while changing so slowly that the change itself is now global news. The restraint is deliberate. Minecraft's real product was never…