The main problem is that the original attractions of Epcot just weren't timeless enough. Take Peter Pan. Yes it's a fairly basic dark ride that's not very exciting if you're over the age of 5, but it's timeless. Everybody knows Peter Pan, everybody wants to take their kid on it. There's no elements other then the ride system that date it to a certain time- ride it in 1980 or 2015 and it's just the same.
It's all very well making attractions about 1980s tech in the 1980s, but 5, even 10 years later, it's already outdated. Even if Disney hadn't removed all the old stuff and kept it running all this time, there would be countless complaints about how they need to update the attractions, why is World Of Motion still using 1980s computers and everyone in Body Wars wearing 1990s haircuts. Future World is the future, not the past.