Problem is, they talked about 400k players, so they must have set their sights on that server capacity.
They didn't pay attention to WoW history. WoW had over 15 million for a good long time.
It would not be a big surprise if they actually sold 1 million copies of the game.
They were not prepared. I remember when WoW ran into this wall, 24 hours, new servers were sprouting left and right. They went out an rushed the equipment overnight to get the capacity increased as quickly as possible. Same scenario for DnD online, so this is not new in the MMO business model, underestimation is a critical error. Shoot, even Rift has a 2 day problem.
This is a week now, early release was the big sign on the wall, but they didn't read it.