And lest people forget or try to argue against that fact, i'll post this again-
That's from August 1989, large crowds and lines for all of the Future World pavilions/rides (all of which were were ridiculous people eaters operating in unison, especially impressive for the park to be so packed). I also personally visited WDW countless times from the early 90's through '97 and similar crowd levels were still seen in busy seasons for classic Future World rides. EPCOT in its original form remained very popular and well loved by guests.
By the mid 90's there were a few things that needed some minor updating here and there (what they did with SSE 1994 for instance was an example of how to PROPERLY upgrade Future World). But nothing so serious that it warranted the complete mess it has been mangled into since the 90's. It wouldn't have been that difficult to keep the park very popular, fresh and up to date.
I'd say it went as far back as 1994 with the loss of the sponsor and choosing not to operate it consistently from then on (it went "seasonal" for a lot of its remaining life and wasn't always open). You can also point to 1994 for the loss of 20k Leagues in MK. But the first major loss for me at EPCOT was World of Motion in 1996. I stopped visiting WDW in 1997 and only rode Test Track in 2010 for the first time, really hate what they did there. Imagination was also butchered in 1998 and I was shocked to see that abomination upon my return as well.
At least Horizons continued to operate intermittently until 1999. I'd say these were remnants of some of the projects intended for "Project Gemini" (which I consider to be a terrible concept personally, thankfully Spaceship Earth was spared such a horrendous fate).