Just because Horizons had some far out tech ideas doesn't mean the other pavilions of Epcot didn't become dated. When I visited Innoventions in my youth I was excited for the next gen games and computers... only to find that my local arcade back home and my desk top computer was way more advanced.
Also, to the point that some have made about GotG being lazy by simply taking over an existing building and not being a new build... well... yes and no. There's going to be a sizable building built to add onto UoE. Epcot is expanding because of GotG.
And with regard to theme: Which theme? The them of "Future World"? In which a history of communication and a sci-fi musical with Michael Jackson and aerial video of California (and World sites) are some of the most beloved attractions of Future World and really aren't themed to the future at all? Besides, our venerable insider has told us that that theme is very likely going to be gone.
Or the theme of "Epcot"? Which was a planned community and then a permanent World's Fair? A fair has fair rides. Not all fair rides are edutainment, like, e.g., Captain Eo. Sometimes... they're just entertainment. Like GotG.