FFXI still has a lot of people paying to play it (every player is a customer), compared to the rest of the market. A 20 year old game with more paying players than many brand new mmorpgs that don't even have a subscription (and in some cases vs completely free games). New world, bless, elyon are examples.
Why is a sub so good for XI.
Firstly from a developer standpoint it maintains a producer > customer perspective, cause we are paying monthly to play the content developers create. Free to play always leads developers into no longer feeling they have to provide a service, which kills motivation to keep making it. Long term, free play kills games from a developer standpoint (even if they make more money short term). I've played plenty of sub games that went free to play, it has always been the ruination of the games over time.
Secondly, a sub tells your audience that you value and still think this is worth it. You're demanding people accept the value in your product, it just works. "yes it's $12 a month, what of it?"
Thirdly, there is more of a relationship between dev and player. I directly feel I'm supporting the developers to make new content than I do when I buy stuff from f2p games. I know my money goes to pay their wages and so I'm far more willing to do it, it's more of a supportive system for people who like the games.
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Thoughts on lowering the sub / XIV joint sub
Offering a lower sub to XIV players:
This would be a really bad idea. Firstly you are setting up a position where you are showing favoritism to one group over another and setting up XIV players as the enemy in your players minds even more than is already the case. The "favored son" if you will. Secondly you're showing less value for people who may have been paying for 20 years, or that pay for multiple XI accounts.
You will be creating bad feeling, where it does not need to exist. Some will quit over this, on principle. I possibly would be one of them, so even if you do bring in some new players....you'll lose others over it. Bad business.
If you want to offer incentive to XIV players, there are better ways. For instance, allow an active XIV player to buy the game for $10 all year around and offer them one months free sub with the purchase. Possibly other things such as XI exclusive minions or cosmetics (since XI was merged with XIV productions and they make all the money from XI anyway, they can afford to make XI themed cosmetics to encourage their players to play both). XIV has a financial incentive to make people play both, XI does not.