Paying a sub monthly is really cheap compared to what a F2P would make you spend. 9 out of 10 items they'd implement in the game would go to the cash shop, because they're all optional after all. All those mounts and minions from achievements? removed and into the cash shop.
Then the RNG boxes kick in, instead of selling items directly, they make boxes with a chance to drop the outfit/mount/minion/whatever you want so you end up spending more. Next would be restrictions on accounts, number of chars you can make, retainers you can have, classes you can unlock, amount of gil you can obtain daily/weekly, unable to use market board or trade with other players which you can bypass if you buy a premium/elite (whatever you wanna call it) service which you pay monthly aswell. You basically end up spending triple or more than you would, and still paying that "loads of cash and wasting money every month/two months/three" sub. But of course, all of that is "optional".
Patches would be delayed several months more, because you're playing for free, they can take all the time they want and you can't complain, because you're again playing "for free". People would lose interest and quit, and less people to do stuff with. Queues would be lovely (/sarcasm).
