Personal experience on attendance it purely subjective, though. There were times when EPCOT was dead and when it was packed during that time. If we went by that I could talk about how no one goes on SSE because of all the times I walked onto it without a line. And how popular If You Had Wings was because there was a wait for it everytime I went to MK.
Here's a big point though. EPCOT was a much different park than it is today. And I think M:S and TT really did help attendance. The thing is, back then, the most significant addition to EPCOT was The Malestorm in 1988. The next big attraction was TT in 1999 and M:S in 2003. That's 11 years without a new attraction. Of course that would hurt the park, which it did. Without thrill rides people considered the park boring. Just like they did AK. That wasn't the fault of either WOM or Horizons, imo.