As someone who was late to the FFXIV train, I just started a few months ago so I started while the unlimited free trial system was already established. But I have many friends who do pay for a sub.
I think their decision to open the game to be an unlimited free trial was a smart move, however their decision to make it a one and done trial was not. I 100% would be ok with paying for multiple months of subscription in order to get the features that the sub provides as a player, but like I know that if I ever chose to do so, I will never be allowed to /not/ pay. This means that me as a player will likely never spend a dime on the game, because I don't want to be introduced into that sunk cost.
If they were planning to go F2P they should have full sent F2P, the hybrid route of "Yea you can play as long as you want, but as soon as you give us money you will be required to give us money forever to play" route is counter productive and is likely costing them quite a bit of money itself.
How does this reflect on the monthly sub cost? Well it's simple, removing the requirement of a sub to play on it's existing userbase, while at first will be a money hit because the non-team players that are trapped with their predatory monetization scheme will leave, but a good majority of their player-base would keep their sub, because FF XIV is very much a team social based game, and the restrictions given for F2P are not viable in later on dungeons as the usage of the marketplace system and the free company system is moreorless a must have in order to play.
Players will swap to a stop the sub when they don't want to play team wise, and renew it when they need socials again, but since the sub is now "optional" there won't be as much of a need to make the sub feel worth the money. Which is their biggest problem right now with other competitors releasing highly successful expansion packs under a fully subbed model.