I think the winner for longest standing structure with no activity would probably go to America Sings at Disneyland. That puppy closed in 1987 or 1988 and all of the stages and seating remained exactly as they were - except for the audio-animatronics, which had migrated to Splash Mountain - until 1997 when they finally began work on Innoventions. When I worked there in 1995, we even had some meetings in it. Truly bad show, on Disney's part, if you ask me...which you didn't.
Oh, and on the subject of Horizons, it was actually open in 1996 when I was there, not just 1997-1999.