I think at the end it realy depends on the amount of Japanese players that are still playing on their PS3. If you have noticed, Japanese games that are released for the PS4/PC only in the West are often also released for PS3 aswell in Japan. This shows that in Japan the PS3 is still being used by a large group of players, if FFXIV has a large group of players from Japan who do not want to upgrade to a PS4, then I doubt Yoshi-P will discontinue the PS3 version of FFXIV of the next expansion.