Concerning the timeline of additions at IoA, you have to remember that right after IoA opened, NBC bought Universal and then spent 5 years trying to sell off the parks. During that time, they not only didn't add anything new, they let maintenance slide into an embarrassing state. The result of this kind of management was they saw their attendance numbers slide from 9 million a year to just over 5 million a year. A slide that even with Potter they have yet to have fully recovered from.
Now, on to Epcot. While Disney seems to be bullet proof in regards to this kind on management style. I believe that the "decade of resting on their laurels" is going to catch up to them and bite them on the ass.