Sneaky Sneaky Grizz...
http://photoalbums.wdwmagic.com/data/500/747a.JPG
Alright, don't shoot me, but based on the option on this poll that MS could have been built with Horizons, it seems extremely possible. The show building seems to fit snugly (without being reworked, although it would, of course, have to be worked on)...and therefore can dub the name "possible."
No, Horizons isn't here physically. But, in order to examine the FUTURE of Disney's changes (i.e. attraction space here, and with JII w/Figment), we must examine the past in order to potentially voice our opinions to bring about progressful change.
As for my personal ideas of Mission: SPACE and Horizons, I believe, although Mission: SPACE is more popular, that Horizons was more beneficial. If Mission: SPACE gave more education about spaceflight - and the concepts of "Dreaming and Doing," it would fit in perfectly with the threads of EPCOT, as well as continuing the legacy of Horizons. Otherwise, Mission: SPACE is an intense ride, but then it's over. I don't feel the "substance."
Anyway, I hope Mission: SPACE will bring great amounts of attendance in order for Disney to raise money to make more "complete" changes.