Completely agree.
I think one of the problems with the original EPCOT Center -- and, in particular, Future World -- was the fact that all of the attractions had a very similar pace. While there were differences in tone -- World of Motion was a light-hearted attraction, whereas Spaceship Earth was serious -- a visitor could easily come away with a feeling of sameness... an overarching sense that EPCOT Center was only about slow-moving, overly didactic experiences.
So change was necessary in order to make the park continue to be relevant and attractive to guests. And I appreciate the injection of thrills into the park, but adding higher-energy experiences need not come at the expense of one of Epcot's underlying, original purposes.
As I've posted in other threads before, you can make an attraction more thrilling without completely obliterating its general focus. For example, the appeal of Test Track lies largely in things that could have been added to the original World of Motion without utterly changing the nature of the attraction's narrative. The TT ride system and vehicles could have been integrated into the WoM story -- with obvious changes to pace and scenes, of course. But most of the things that make TT appealing -- the changes in speed, the hills, etc. -- could all be made part of the WoM's tongue-in-cheek history of transportation. The exposures to heat, cold, and "corrosive" acid could have become part of the story of Americans' westward journey, and the extremes of climate that wagon trains endured; the "rough" road sections, swerving, and narrow accident avoidance could have become part of a segment on the expansion of the interstate highway system, and the growing pains the U.S. (and other countries) went through in transitioning into a car-focused society. Lastly, the high-speed outside loop could have been added as part of a way to "slingshot" guests into one last show scene depicting a potential future of transportation.
Put another way, Test Track's appeal is not primarily due to its theme of automobile safety testing. If anything, that theme is way too specific -- a point you make well with regard to Mission Space. I'd really rather not have an entire Future World with attractions dedicated to very narrow topics within the general ideas of "transportation," "space exploration," "communication," etc. -- not when the attractions can, if designed well, address much broader scopes (the big ideas that inspire people to explore and create) while also providing thrills: the two are not mutually exclusive.