Dissidia was killed outside of Japan because of huge amounts of Japanese exclusive content. I wanted to see Rinoa in KH, 14 years after the KHII tease. It's ultimately a sign that Kingdom Hearts will be a Disney only game series going forward.
They should have released the game finished. KHIII has a few major issues, that are fixeable through patches and DLC. This game deserves FFXV's level of support, since it isn't a trainwreck like FFXV and can be fixed.
I love how people complain about Kairi not Aqua. Kairi has always been useless but Aqua jobbed badly against Vanitas and she stood there for half a minute after Terranort murked Ven. Xion replaced Aqua as my favorite character after III, largely because of this.