Honestly, the way you guys talk, it's like you really don't know all that much about creative writing at all.
If you need multiple solo films to make the audience care about the characters in a movie, you're not doing it right. Every single movie - whether it be a shared universe or separate, whether there is solo movies or if they're not - should be able to make the audience care and feel emotionally invested in all of the main characters. That's just the backbone of good writing.
And yes, it's hard to do. And yes, there's plenty examples of ensamble films that weren't able to pull off a good balance of characters (Singer's X-Men, for instance), but that certainly doesn't mean we should just assume a JLA movie should be inherently bad because it will actually take skill to write it.
If we're relegated to that brand of logic, then even with a shared universe, Avengers would've been a Tony and Cap lovefest where we didn't give a **** about anyone else. But it wasn't. It devoted an appropriate amount of screen time to every main character and gave them all individual character arcs in the film to make the audience care. And that's what should be done with a JLA movie.
Also, the reason why the solo movies were so important to the Avengers movie, was not for creative reasons, it was for financial ones. Before 2008, not a single character in the Avengers could be said to be a bankable, financially successful property. They needed to make sure that the GA was open to and responded well to IM, Thor, etc. before they could stake such an expensive film on them.
For WB, they don't need to worry about that. Both Superman and Batman have proven themselves time and time again to be bankable comic book characters, able to support big budget movies. Their presence alone would guarantee that the GA would at least be interested in a JLA film (obviously, whether it's a hit or not would be dependent on its quality).