There are some obvious ebbs and flows in MMOs, and that's simply because you can't be in two places at once. I would reckon that there will probably be a temporary loss of players in FFXIV just as I would expect there to be a loss of players in other MMOs when the expansion for FFXIV comes out. Players like to have new content in their games, and the promise of new adventure is always alluring to those who speed run through content.

On a side note, Warlords of Draenor is probably one of the lack luster expansions that I've seen. Not only does it feel like another Cataclysm expansion due to there not being a new land to discover, but there aren't even any new Classes/Races to draw people back in. Everyone I know who used to play the game, myself included, hated the fact that you can literally skip straight to level 90 and play new content as a new player. It takes away from the player who worked hard to accomplish everything they've done, and being put into a party with a healer who doesn't know how to heal or a tank who doesn't know how to tank in a 90+ dungeon is not the way to keep your experienced players happy. It's for this reason that I would give players who return to WoW a month before they've completely geared themselves out and raised all their alts to 100 and realize they have 5 more months before Blizzard releases any worthwhile patches.

I would actually think that due to a lot of console players populating the game that the release of FFXV will actually hurt the population of FFXIV more than WoW ever could.