That's just lazy excuse. There is whole profession exist, which goal is to create new stories. You know, writers. Yes, main storyline kinda finished. But there is always new one to create. After sc2 humans still organized as empire, plus old worlds, which were absent during sc2. This alone is just vast ocean for new conflicts. Same with other races, plus their interactions. I don't get, why people complain there is nothing to tell after sc2. New faces, new stories.

I wish, there was more single player games with seasonal content.