It has failed it's original theme and purpose, but it has not failed as a theme park. It could use some changes and updates, but it is still a great park to visit for a full day experience.
They do well with Food & Wine and Flower & Garden Festival.
As a theme park it has been a bit stagnant.
If you think about Universe of Energy, how many people go in there for a break, to take a nap, or find that they've taken a nap. I like the attraction, but I can't think of any other attraction that actually has that affect on people. Sure, during the hot Summer people will go to the indoor attractions to get out of the heat or head to attractions where they can just take a break, but you don't often find them napping in on those or seeking an attraction out for a nap.
The Living Seas feels like a Nemo retread to me. There isn't much of a story except the father yelling, "NEMO! NEMO!" over and over again. There are some neat effects with the characters swimming in a 3D sort of way in the aquarium but that's about it. It's not a very engaging ride. The outside entrance and queue area are well done. The ride and then the inside, which looks like Sealab 2020, is pretty blah. I've never had a problem just walking onto this ride, even on the busiest of days. I will attribute some of that to it's awkward location.
Imagination is pretty blah. Everyone knows that. It seems like with a subject like "imagination" that it should be awesome, but it's not. It's a bunch of parlor tricks at the beginning and then crazy land for the 2nd half.
I think that The Land had held up well, as has Spaceship Earth. Test Track is definitely popular and Mission: Space is not as popular as you'd think it'd be. It's almost always a walk-on for me.
Innoventions is OK, but something I normally just skip.
I really like World Showcase but it seems like, after 30 years, they ought to add a few more countries and attractions. Finish Germany with the Rhine River ride. Add in Australia, Spain, or Russia. Something new back there would be nice. The Frozen makeover could be good or it could feel like a retread.
I like Epcot, don't get me wrong, it's just that it could use some love (more than retreads) and it's not so much that it needs a thrill ride, but more that it needs more interesting attractions, updated films, etc.