EPCOT also lost it's popularity with that form as well. It cut both ways. Sure, it opened as a success and stayed that way for awhile, but there's a reason the park started shifting toward where we are now years ago. People stopped caring. We can't change the history. The decision to add rides like Mission: Space and Test Track didn't come from nowhere.
I understand that OG EPCOT fans really, really loved it. I am sure I would have too. EPCOT is my favorite park on property, and I'm sure it would've been at that time as well.
But we've all got to stop pretending that OG EPCOT would survive in the modern world. It was becoming less and less of a point of interest and relevance even in the world of the 90s. And yes, its lack of variety was very much a part of it. I think a lot of OG EPCOT fans have gotten it into their minds that their love of the park and everyone else's love of the park was the same, and it just isn't true. Yes, people loved EPCOT as it was when it opened because when it opened, it offered something insane and new that everyone was blown away by. Fast forward 15-20 years? OG EPCOT fans were still there, but everyone else had moved on.
The evidence that people started to find it boring is there in the response Disney gave to it. Like I said, the idea to add thrill rides didn't come from nothing. It came in response to people starting to get bored of the same ride system used over and over again, even if the attractions themselves were quality.
What was good for audiences at that time was not what was good for audiences down the line. People's tastes and interests changed, and EPCOT felt that change before they started moving toward offering different types of attractions.