Based on the content and scale of XV I'll like it to The Witcher which spans 3 games and In now way to sequels hurt it. In fact the bolstered the story greatly. This game is coming from Nomura too, and people herald the coming of new KH games.
Sequels in general are getting a bad rap due to the way the video game industry is using them (aka anything the least bit successful gets an unplanned sequel no matter how much the story and quality of the game suffers).
Some stories need multiple sequels to tell their story (Mass Effect, The Witcher, etc). Some stories are are the kind that are left open to multiple entries down the line (Bayonetta, Devil May Cry, Resonance of Fate). Some stories, like Final Fantasy XIII, honestly work better as a stand alone due to the nature of the ending and overall plot. That's not to say that an XIII sequel
couldn't have worked, only that a hell of a lot more thought and effort than what was given needed to be put into making it. And then some stories, like The Last of Us, are meant to be single entry titles. Any sequel to a title like The Last of Us will, 99% of the time, diminish the impact of the original title.
The fact that Final Fantasy XV is being made with the intention of expanding the story in mind is encouraging to me. It means that the people involved accept the reality and inevitability of sequels due to the current state of the industry and the massive scale of the project and are learning from past mistakes. It means that the games will
hopefully be more cohesive as a whole and that there will be a lot more continuity. Most of all, though, it gives me hope that we will get a fleshed out universe unlike what we got with XIII.
Whether or not XV will deliver on all this remains to be seen, but what I'm hearing has made me hopeful.