Fantastic Beasts producer David Heyman recently discussed the decision to extend the series to five movies from an initially planned trilogy.
In an interview with Collider, Heyman responded to a question about the extension, saying “It wasn’t a difficult conversation to have at the studio… it didn’t go from anybody speaking to Jo
and saying, ‘You know what? Let’s milk this.’ Both in fairness to the studio, but also to Jo. While it may look like it to some people, there is nothing cynical about this. This is all from her head.”
“So she begins with three films, because she thinks that’s the story she wants to tell, and then as she digs deeper… and she hadn’t written anything when she said three. Then she wrote the first, and as she was writing the second, actually just before then, but as she says we working on the first, she began to realize there was a whole lot more, and she was trying to figure out, ‘how the hell am I going to squeeze this into three?’”
Source: IGN.com Fantastic Beasts Producer Defends Decision to Extend From 3 to 5 Movies