What lesson? Avengers made more money domestically in 6 days than JL did in its entire global run. That should more than serve as a wake up call. DC should have long fell back to the core values of what makes their characters work instead of relying on a "visionary artist" like Snyder.
What bums me out the most is how often I see Snyder posting "behind the scenes", "deleted scenes" or "concept art" on an almost weekly basis, or offering interpretations on the hidden meaning of his scenes all to flame the fanboys fires in a false belief of "what could have been great".
What's done is done. It looks like neither DC nor Snyder could take a deep look at themselves and say, "Yeah, we messed up" but instead rely on the blind fanboys that will support anything they do to for reassurance. Stop blaming others and own up to your mistakes- y'know, like how superheroes would do? Until that has been addressed, we'll never get a half decent DC movie.