Don't get me wrong, I love the OmniMover. It is a shame that they are slowly dying out of Epcot because they are a versatile ride system that can tell a good story and maintain a high capacity. All that said, it was time for Horizons to go. Even if it had been updated in 1993, it would have been outdated by 1999, let alone 2009. Look at some of the attractions in Epcot today that are from the same period that a Horizons update would have been: HISTA (1994) is desperately in need of replacement; Ellen's Energy Adventure (1996) is still a good show, but the films coud use an update. The 1994 version of SSE wasn't even changed until 2007. The trend here is that attractions opened or renovated in the 1990s haven't seen (m)any updates, even up to today. Even if Horizons had been updated and Mission:SPACE had never opened, we'd be stuck here today with an even more outdated ride and another dead corner of Future World. What happened, happened, and it was probably for the best. Even if Horizons operated as an out-of-date, out-of-touch attraction several times in the 90s, it is out of its misery now, and has been replaced by a much more advanced ride with a broad appeal. M:S has nothing on Horizons' capacity I'm sure, but it also helps open up Epcot to a wider audience.
Bringing a message of innovation and inspiration to a new audience through new technology and quality entertainment. Sounds like Mission: SPACe fits pretty well with the "original" mission of Epcot.