Now, is it reasonable to talk about an "Indiana Jones Trilogy" that is distinct from Crystal Skull (and any subsequent films)? I think it is. After all, we're clearly allowed to talk about an original Star Wars trilogy that is distinct from the prequel trilogy. The first three films were released from 1981-1989, and then Crystal Skull came a full 19 years later in 2008. For reference, there was only a 16-year gap between Return of the Jedi and Phantom Menace.