Don't forget Big N pushes out new series as well. It might not be as -big- as the veteran series, but it's refreshing ie. in the GC era there was Pikmin and Animal Crossing.
I don't think there will be ever a "enough" as the games that gets revived are great games. If people buy, they will make. FF is up to its XIII installation because people just love them. Not many will want SE to drop FF already! Though true each game is pretty much unique they are all based upon a RPG system with good FMVs. Like SE, Big N pleases fans of its series, if they drop it, it sorta feels like your favorite band decides to quit making music : /