This isn't new. Kingdom Hearts II only had like Sephiroth in its post game. DDD only had Julius.
And I'm pretty sure that BBS had basically no post game also, I mean, besides the Arena but that isn't a post game, it's something you can access anytime.
Let's just hope KH3FM is a massive expansion, like a whole new endgame world kinda massive. Like a Cavern of Remembrance blown up to a whole new world.
Cause I'm pretty sure hardcore fans spend the majority of their playtime on endgame content.