The only actual additions I can think of this century are Expedition Everest, Tron, and Ratatouille. That's off the top of my head, though, and I wouldn't be surprised if I'm forgetting something. Everything else was some sort of replacement rather than a completely new build in a previously unused area.
To be fair, some of them were replacements for areas that no longer served a purpose. Although I think the original Backlot Studio Tour was 1000x better than anything in Toy Story Land, that whole area was essentially obsolete once they stopped using the studios for actual animation and filming. I also don't think anyone wishes Camp Minnie-Mickey was still there instead of Pandora.
My understanding was that Camp Minnie-Mickey was essentially a placeholder when the original Animal Kingdom build was scaled back. So I would not consider Pandora a "replacement" -- it was new and expansion.
Similarly, I would consider the DHS Star Wars expansion as a new expansion, it merely replaced the stunt show.
On the other hand, the Backlot Studio Tour and The Great Movie Ride were both formerly core DHS attractions that were "replaced" by Toy Story Land and Runaway Railroad.
Fantasyland Expansion was an actual expansion.... Yes, Snow White's Scary Adventures closed, but a whole lot more opened: The Mine Train, Under the Sea, Fairytale Hall, Pete's Silly Sideshow, Enchanted Tales With Belle.
So unfair to say there have been no "additions" this century. I'd say there have been quite a few--- except in Epcot, until the new France expansion opens.