We are kind of seeing this with DRG right now, and the response from SE has been to announce a buff to DRG, keeping them relevant (hopefully).

NIN was a solid addition to the game, I think most people will agree. But even if they do have some jobs just completely fall off the way-side, this game would be awesome to have to reroll, since gear is reset every 3 months pretty much (new iLVLs are given in .0, .2, .4, while super easy ways to get that gear is introduced in .1, .3, .5). Its super quick to catch up in gear to a reasonable degree in this game.