I interpret "worse" in terms of a thematic fit, not necessarily the quality of any existing ride.
I don't think it's anything to do with Tomorrowland. They're already spending some time and money on CoP and removing Stitch, Buzz or Monsters Inc. would be an improvement. Same goes for almost anything else at DHS.
My guess is Future World. UoE, Wonders of Life and Innoventions give plenty of dead or underutalized space to work with, and few things are more irrelevant to the land's intended purpose than hyping a comic book blockbuster.
Mission: Space would make for a easier overlay, but the ride's somewhat popular as is and doesn't really need the same kind of attendence boost as other Epcot attractions.