It depends on how many subscribers it will get, and what the retention rate or growth is. And that all depends on how aggressive SE is with marketing, welcome-back campaigns, in-game services, and offerings outside of the MMO (like phone apps).

RIFT and WOW stay in business on a subscription model because they both have awesome phone apps, content accessible through a web browser, developer blogs and very frequent developer-community interaction, paid character services, and extremely aggressive recruiting campaigns and advertisements.

TERA, SWTOR, and all these other MMOs went free-to-play because they had none or very little of that.

If SE realizes this and starts doing the same, FFXIV will have a long, prosperous, and subscription-based life ahead of it.