Just like MK can't add more than 17.4 million.. what's that? Added a million guests with NFL? My bad.
It's short sighted for you to say that, especially when Universal is getting ready to expand their family friendly options. Both parks could probably hold 11-12 million a year at maximum capacity right now... add in the expansions/revamps, they could hold quite a few guests.
It's not speculation to assume that they'll each have 10 million guests by 2021. IOA has 7 years to add the 2 million necessary. USF will have 6 years for the 1.5-2 million necessary. Both doable if they keep up the current pace.