Except he has no clue what the market for the most popular and lucrative MMO on the planet needs. He has never, ever, ever worked with a game of that scope.
Guess what? You take someone with a much smaller scope than is necessary and plop them in the middle of this Goliath of a game, you will see a dip in quality. Period. It happens all the time in video game development, albeit to a different degree of success. Check any studio which sees massive success of one of their titles over the others. Designers, engineers, and others involved in the process will either need to scale appropriately in talent and scope or they will be forced to replace their team with people more apt for the job