STS doesn't release numbers but I doubt more than 15% of the player base buys plat. That 15% (or whaever it is) has to generate enough money to support the staff, servers, developers, etc....The entire game is designed around buying plat, it can't change to a subscription based game without being a different game.

Free games that are fun to play are hard to find, especially an MMO. AL can be certainly be played without spending a dime if you are good at making friends and have the time to put into it. If that is changed to monthly fee, the number of new players coming into the game will drop off dramatically, plus a huge exodus of existing players. STS needs that flow of new players constantly coming in in order to have new people buying plat.

Anyway that's my theory