Why do people think that if DC uses Darkseid first, then they won't have any villains to use later?
They could do an Atlantean Invasion story. They could do a story with a secret and shadowy version of the LoD/Injustice League/The Light like in Young Justice. They could use Hades. They could even make Felix Faust a servant and worshiper of the inter-dimensional god Starro (just make him a Lovecraftian as possible). It's not hard coming up with threats. Just read Grant Morrison's run for other ideas.
It is not that somehow there will not be any villain to use, it is just that you cannot top Darkseid, the ultimate threat. If Justice League is to become a franchise, that is going to be huge problem.
Anti-Monitor. Crisis of Infinite Earths. Explore New Genesis and Apokolips further in a sequel. Black Lanterns. There are plenty of ways to top Darkseid.
1)Those are not Justice League stories.
2)There is just no way to make a movie out of Crisis on Infinite Earths because it requires dozens of movies about those Earths otherwise people would not care about them disappearing. Same thing about the Black Lanterns, which would require hundreds of superhero deaths prior to the actual movie.