Assuming they were selling out and people were being turned away.... but they aren't and never have been. More attractions means more places to put the existing people which will lower overall waits and improve satisfaction.
The new thing will certainly create an attendance pop over the first 6 to 12 months but it would then die off.
All i can say is that based on the history of all previous add ons over the last decade+, and crowd levels over that time period. i don't see where your logic comes from.
If you do out the math, WDW has over 36K rooms, assuming a 75% capacity with a median of 3 people per room, you are looking at 81K people per day, from what i could find it has been reported that around 30-40% of guest stay on site, and 60-70% of guests are offsite, based on that you're looking at an estimated 150K more. Grand total of 231K guests per day. That is filling roughly 60% of the parks daily, not considering Disney Springs, either water park, or people who just spend the day at their hotel.
That being said, i cannot see how adding more parks would help, they are already bursting at the seems, and realistically probably only at 50% capacity on an average day. New parks will also come with more hotel offerings, onsite, and off, and a boost in tourism to central Florida for the foreseeable future, (Florida tourism has reported a boost in tourism to central Florida every year since the great recession, so saying that Disney won't see more and more guests is a little foolish based on travel patterns) all adding up to the same issue the parks are currently facing.
I would hope that Disney tries to utilize technology over the next decade to attempt to quell the long lines in their parks, i would love a 5th gate, but i just can't see that as a solution to an over decades long problem now
