Harry Potter books and movies have such a big following, and fans can't wait to see or read something related to their hero. They might not like it, or they might be disappointed by Fantastic Beasts as a movie, but they will still go and see it thus resulting in good results for the franchise.
Nobody is going to ever admit when a franchise is played out but as a person who has never been a real fan of the series to begin with, I think the time is now to fold -- but that won't happen. I know the same can be said of The Simpsons or South Park, but I think they both continue to prove their worth more than the Harry Potter series.
Sequels fail. They fail a lot. The charm of the early Harry Potter movies was borne more out of the youthful innocence of the characters. Now, they are older, and the series seems to have become more of an adult contemporary run of films, as opposed to the action/adventure series that was meant for younger audiences.
I do not believe that the Fantastic Beasts movie will live up to the hype. Even though Fantastic Beasts was written by J.K. Rowling and is part of Harry Potter's magical wizarding world, it's not the same thing. The characters are different, the storyline is different, yet it's getting the same amount of hype. So I question whether it'll be able to live up to that hype, which I don't believe it will. Too many people will go see it thinking Harry Potter is in it and be disappointed.