What we do know is that 10 years from now, the franchise that started this fall will still be going on, as such, people will be watching this movie in the future if only to prep for the final installment, which will be hitting screens just about 10 years from now.
We honestly have no idea if Fantastic Beasts and Where to Find Them will have that special something.
In general, the reason that people still watch a movie years after it came out is because something about the movie is timeless and resonates with people in any era.