I dunno, I feel like WB/everyone being "taken aback" by the critical response and scrambling about what to do with Snyder is inaccurate. We didn't start getting reviews of BvS until a few days before release - they were embargoed until then. Now, they obviously didn't know the critical response would be THAT bad, otherwise they would have held the embargo until
the day of release, which studios have done before.
But WB has also been on the opposite end with TDK, where they let the reviews fly in
two full weeks before release, because they knew they had something magical. (Marvel is experiencing the same thing right now with Civil War.) Studios know when they have lightning in a bottle or a dud on their hands, they're not as clueless as we take them to be.
Now, it obviously isn't smart that WB scheduled JL to start filming mere days after BvS's release, before seeing what the reception would be. But they must have felt that they had no choice (for whatever reason) and it was a bet they were willing to make. You don't jump whole hog into something like that without considering the consequences, unless you had a TDK-type guaranteed phenomenon.
They know. They took a bet, and that's why they're sticking to Snyder (at least for the moment) - it was a possibility they foresaw and they decided it was something they were willing to risk.
Now come the matter of, how to solve the previous movie's issues, or if the suits are going to start freaking out and shaking up the filming/release schedule left and right.