Are we really having an argument on here about what makes an attraction modern?
Looking at Epcot, it really only has two rides that always have long waits (Soarin' and Test Track). The advantage that Epcot has, is the capacity of that park (with all of the restaurants and rides/attractions) often exceeds the demand. Epcot draws 2/3rds of the crowds that the Magic Kingdom does, and it seems that it's capacity is comparable. The result is shorter lines for it's attactions.
The target audience (children/families) for The Seas with Nemo and Friends, probably spends more time in Fantasyland on their average vacation than they do in Future World. For that reason, if The Seas with Nemo and Friends was in Fantasyland, it would generate longer waits.