Buying a Lifetime Subscription is effectively betting that you feel the game will still be worth playing years down the line. If indeed you do feel that FFXIV will still be immense years down the line, buy shares in Square-Enix, because that way you will be able to profit on the boost in share value they will receive should this game become as amazing as you feel it will. That way, you will effectively have the increasing share price offset the cost of your subscription.

Tada!

I think SE are in this for the long haul, and with that in mind, I doubt they will offer Lifetime Subscriptions. Annual subscriptions, though, that's another story - I wouldn't be against them.