That is, of course, if the game is actually coming to PC at all.
Obviously it is. In case you didn't notice, Square Enix has been releasing all of the main-series Final Fantasy games on the PC over the last few years. With the exception of FFXII, which is getting a remaster release on the PS4 this summer and will likely come to Steam a bit later, all the numbered Final Fantasy games from III to XIV are already available on Steam. As such, it seems pretty obvious that they've been planning for XV to come to Steam as well, even prior to its console launch.