The game isn't dead. It doesn't draw in a lot of new players, but the existing player base is big enough to support the game for a long time.
Taking down the servers would be complete lunacy, seeing as the investment of developing the game has paid itself off years ago, and every single month generates a substantial profit for them. At this point, I don't see the game actually being shut down in the next 5 years. We might only have a single server left at that point, but even that would generate enough revenue to pay for server maintenance, while still leaving a small profit for the company.
As for the amount of subscribers, I think 500k is an overstatement. The actual amount of subscribers is probably somewhere around 200-300k. However, this amount would still generate a revenue of around 40 million dollars per year. Now I'm no MMO developer, but I doubt a game like FF11 costs 40 million dollars a year to maintain. The game is already paid for, the only thing they need to spend these 40 millions on is server maintenance/equipment, a few GMs and community representatives. Everything that's not used on that is pure profit.
There's probably just a handful of developers left for the game right now, with probably around another 10-15 people who work as GMs/community reps, and translators. The actual server hardware for FF11's servers is probably not extreme either. The server software is 10 years old, so it probably doesn't require a lot of resources relative to how powerful server hardware today can be. The game doesn't use a lot of bandwidth per client either. I cannot possibly see how this would cost more than 20 million dollars a year (and even that is probably way more than what it actually costs).