You don't pay at all to play Mahjong...so what's your argument here exactly?!
Not only do you pay the sub to play Final Fantasy XIV, with Mahjong just being on the side (would you pay a monthly subscription for the Gate's?! For Chocobo Racing?! For ANY mini-game for that matter?!). There's also a permanent free trial that allows you to play it without spending a dime (technically...so long as you do not actually have a customer account, that is). This argument is kind of nonsensical.
Huh?!...Did you have to pay to get Mahjong?! No...So what are you on about?! It's not a paid DLC. It's part of the core game that everyone that can play Final Fantasy XIV A Realm Reborn in any capacity can use. It is exactly the Gwent that was available in Witcher 3 from the get-go. It's the Triple Triad that was in Final Fantasy VIII from the get go, Blitzball from Final Fantasy X etc.
This game is a subscription based game. That's a fact that ain't gonna change in a foreseeable future. Mahjong is an addition to all that you can do, not a replacement. It's a value added.
Your arguments are in no way relevant to Mahjong as it is implemented whatsoever...
