Although I agree ending on Terra could have been good IF done well; you simply need that Final Dungeon in FF. Well, at least you used to.

And Memoria is one of the best. If you played other FF's (or better yet all) it signifies that everything comes from memories, and those memories are embodied in the crystal, to me disc 4 is the icing on the cake - this game was a reward for FF fans (yet you don't need to be a vet to enjoy).

Necron, like other parts of the game and final dungeon, also has other FF's as his foundation. The system to end all existence. An artificial imbalance of light and darkness took place, and there he is. Yet hope comes out as the victor in the end. That's just Final Fantasy for you.