I wouldn't hold my breath for a lifetime subscription option as it just doesn't make financial sense for most players or SQUARE ENIX. The fact that most MMO's run until they literally can't anymore precludes a company from being able to price an option like this smartly. Look at FFXI, it just entered its 10th year and its still going.

Besides, if SQUARE ENIX did provide you with the option, what would you do after shelling out the ridiculous sum of cash and either A) the service was shut down or B) you stopped playing for a while (or even entirely)? You would be out the money with little to no recourse in recovering it. I would certainly imagine that there would be an "All Sales are Final" clause in the agreement for something like a lifetime subscription.

Bottom line: a lifetime subscription just doesn't make sense.