People really don't seem to grasp this 'free game makes more than subscription game' thing, but it's absolutely true, and has proven such with MANY companies. Even games with very small userbases are still around, because they're still making money.
Let's say SE has 100,000 paying user at the moment. This is a 100% Conversion rate, with 12.95$ ARPPU (Average Revenue per Paying User). The ARPPU is probably higher, but let's just use the lower amount per month, for fairness. That makes $1,295,000 a month.
If we go by industry standards of 5% Conversion rate and 25$ ARPPU (So some people pay 1$, some pay 500$, but you're not in the ARPPU unless you pay something), you'd need 1,036,000 Monthly user to generate the same income as they're currently getting. (Or about 51,800 paying users)
Of course, that would also mean you need to find some way to generate another 25$ out of your users again the next month.
Or you can be happy with 100k subscribers, get the same amount of money monthly for next to no effort.
I can see why PEOPLE may want FFXI to turn Free to Play, but I can't see why Square would do that move --- they're making 1.2 million out of thin air every month.