Dick being the only hope is hyperbole, but I can see their point: the combination of killing him and stuff like shooting Jimmy's face off and Superman's "symbol of hope" status being lip service at most is all a bit much right out of the gate.
Dick and Jimmy in particular are bright spots in their respective franchises that humanize Batman and Superman in ways no other supporting character can quite replicate, so icing them in a shared cinematic universe that is trying to spread the message of "light in the darkness" seems counterproductive right from the start. You just took away Bruce's greatest achievement as a hero in someone else's life and Superman's best friend because he can't have a life outside of Lois and the JL apparently. But they saved the world from another ugly CGI alien invasion, so I guess things are hopeful?