Everyone knows Fantastic Beasts is a part of Harry Potter, maybe not the original story but they know it's a Harry Potter thing.
Harry Potter still has quite a bit going for it, in part thanks to the parks. It's got a play coming out in the summer of 16' as well as continuing spin offs and info from the stories' world. There is still a massive, and somewhat slowly growing fan base. I can guarantee you it will have lasting power.
That said I certainly believe that Star Wars is the definitive answer to Potter and it will signal a beginning to a new theme park war - if Disney is willing to get into it, and with this announcement it appears that they are.