^The first movie they'd probably put out is a Batman one. Followed by Superman.Bad argument. The reality is, if JL Mortal had come out and bombed, it would be treated like Singer's Superman and promptly buried forever.
Now, if it had been a hit? You've got a different set of circumstances. Then, yes, they would've done like Fox did with Daredevil (and hoped to do with FF): spinoff movies (like Elektra and the aborted Silver Surfer project). But the fact is, it never happened.
Robinov is the head of Warner Brothers with, as L.A. Times stated, "ultimate say" to make a movie happen. And he shocked everyone by going on record with his desire to do a Justice League movie. You're using what-ifs to make your case instead of using the facts--and the fact is, Robinov wants this. His team of executives is trying to figure out how to do this. The only way it's not happening is if they all put their heads together and come to the same conclusion, "No, Jeff, this isn't working. We can't find a way to do this that would guarantee minimal success. Let it go and just stick with Superman and Batman."
Using the what-ifs you're using, Marvel would have stopped trying to get movies made 20 years ago after FOUR humongous bombs and colossal failures: Lundgren's Punisher, Corman's FF, Salinger's Captain America, James Cameron's Spider-Man. They didn't. They kept trying.
If your case is that a Justice League movie can't be done for the reasons you've stated or in the way you've stated, that isn't going to stop them from trying. Just the opposite--and Marvel's continuing success as a box office juggernaut is why. As a matter of fact, the more Marvel movies we get? The more WB (and others) get green with envy and try to copy what they've done.
The Avengers movie and its likely success is going to embolden WB to make Justice League, not dissuade them. They certainly won't do Wonder Woman first; it's too risky and they've never been able to figure out how to do it. Flash has a very outside chance as an Iron Man/Thor/Cap-type of bridge to a Justice League movie *if* WB decides to follow the Marvel model. There is a very credible argument to be made for a Flash movie as a lead-in to a JL movie instead of using GL2 for that.
But the only way it's going to be WW is if Nicholas Refn has a huge box office hit with his next film and then champions his idea for a WW project. That's the only real shot WW has right now; it's probably going to be GL2 or Flash.