Ah, Horizons... I sure do miss it. Early in this thread, the comparison was made to rides like Pirates and Haunted Mansion, which haven't needed changes to remain popular. Those rides involve telling a specific story. While there have been enhancements, the story is largely the same. In Epcot's Future World - a park designed to showcase the latest technology - it stands to reason that UPDATES will regularly be needed.
A large component of Horizons was devoted to how previous generations viewed the "future." A substantial portion of that ride dealt with "the future from the 50s." When Epcot opened, those who fondly remembered the 50s were age 30 and up... just the right age to be visiting WDW and bringing their families. Now it is only us oldsters who remember the 50s. (An aside: How much longer will Prime Time Cafe maintain its appeal to growing numbers of visitors who don't remember what it was like to sit around a kitchen table watching black and white TV sitcoms?)
As others have noted, our vision of the future is always evolving. That has been the challenge for imagineers to keep Future World relevant in an ever changing world.
If I might toss out another example: I love The American Adventure. However, the final scene involving audioanimatronic figures is devoted to World War 2. When Epcot opened in 1982, WW2 was still within the direct memory of anyone over the age of 45. In 2018, we are 73 years past that war. While the ending film has been updated to incorporate more recent events, I'd like to suggest that events like the moon landing, the dawn of the information age, and advances in medicine would be worthy elements to incorporate into the main portion of the show.
Shifting gears again, I don't mind the injection of Disney IP into Epcot, but I would rather have the Finding Nemo characters used to articulate the wonders and mystery of "The Seas" than to repeat the story of Nemo we can see by watching the DVD of the movie.