Sorry, but 'greatness' is not an admissible reason. John Stewart was featured in JLU, but that hardly makes him an accomplished and popular character. It'd be like me proclaiming that Brandon Stokely was an elite WR just because of Peyton Manning. No dice. Stewart got his 15 minutes of fame but he hasn't done anything noteworthy to dethrone Hal Jordan (and never will) as the most prominent Green Lantern.
Another anemic reason.
The general audience isn't coming to see Justice League for Flash, Wonder Woman, Aquaman, Martian Manhunter and Green Lantern. They'll be forming lines for Batman and Superman sharing the same screen for the first time in history. The usage of Hal Jordan won't affect the Box Office, and if you believe that, then you're ignorant as hell.
I seem to recall a small faction of fans belittling Marvel Studios and Joss Whedon for their decision to use Hulk in the line-up after The Incredible Hulk underperformed. A re-cast and two years later, Hulk steals the show and The Avengers breaks box office records.
Moot point, period.
For the Trillionth time, because John Stewart was not a founding member, and nor was he the first Green Lantern.
And great.

I'm sure Justice League is going to succeed with or without your money anyways, so do what you want.