FFXI has been SE's most profitable final fantasy title ever. And while we don't have any numbers, from bits and pieces it doesn't sound like this game takes much to keep running, so I would definitely agree that even with a smaller population then we have now, FFXI could still turn a profit.

But given it's history as a money maker, and it's devoted fanbase, I don't think they're going to cut the cord anytime soon.
Though we all know, there's a fair bit they could do to improve FFXI's population, but that of course, takes a lot of money.

I think an engine/graphical overhaul/update would do wonders, but that's essentially re-writing the entire game, and we know that comes with a steep cost.