Oh, for sure, boxes do end. But there's a long tail. e.g. Diablo 3 sold 12 million in the first year, 3 million more in the next year. A lot less.. but still a lot of boxes!!
If Overwwatch made $500m in three months I have no trouble believing it could make, say, $700m by year's end. And that could be getting into the territory of challenging WoW for this year's biggest earner.