I think the real problem lies in the fact that FFXIV already acts a lot like a single-player game, so it's stuck competing with other single-player games. I probably won't be logging in as often when Fallout 4 comes out...but I'll still be logging in. The people that will be hit the hardest will be those who are already burned out and tired of the repetitive grind.
After enough patches, players will come back. Those that don't will certainly do so when the next expansion releases. SE won't lose too many subscribers in the long run, but there'll likely be a dip in their numbers.
...Now, when Persona 5 comes out, none of you will be seeing me for a very, very long time. >=]