I would respectfully disagree. At absolute worst, the number of "paid for" subscriptions would stay the same. On the other hand, the OP brings up the valid point that it definitely speeds up and simplifies the process of "paying", because it's an in-game mechanism rather than using the unequivocally terrible Square Enix Account Management System. This would make it easier, not harder for people to impulse-buy subs, and if Amazon One-click is any indicator, this would lead to more sales.
The lower the price that the subscription-item costs, the harder it is for RMT to profit from the game, so market competition (or undercutting, as you put it) here is good. It also lowers the amount that PLEX-buyers can make from spending cash, which is also good for "fairness". Also, the "F2P option" here is certainly NOT available to any 12-yo with internet access because it requires time and dedication to actually make the money needed to buy the subscription-item.2.) It allows for a F2P option in this game. Not only does this open the community up to every 12 year old with internet access, it also encourages (even more than now if humanly possible) greed and competition between players and exploitation of game resources. (If you think RMT are bad, wait until you have 500 other players undercutting everything you try to sell to pay for their subscription.)
Please see the last paragraph of my post.3.) It does nothing to stop RMT, absolutely positively nothing. If they can't make money selling game currency and U/U drops are the best gear in the game, you think they'll simply throw up their hands and say "whelp, we can't sell currency anymore, time to leave" instead of, oh I don't know, SELLING THE U/U DROPS maybe? RMT can and will sell anything in any game that players put a value on. If SE removes the value from gil, they'll just switch to selling items and powerleveling. That you believe RMT will somehow magically go away, or even decrease in number, is naive to say the very least. RMT don't care about how they make money, they will find a way to make money.