The "dark and gritty superhero" formula is not sustainable

This is correct.  Outside of actually testing out the whole shared movie universe concept, Marvel isn't really trying anything groundbreaking (as far as individual movies go).  They stay in the pocket.  They have a particular tone and four-quadrant formula that they want to execute in every movie, and then they hire people who are competent enough to carry out that vision.

The problem with BvS is that the theatrical cut was a mess.  And when you couple a messy script with an overly serious tone, you open yourself to ruthless criticism.  Had they played things a little more safe (in terms of tone), I think the criticism wouldn't have been as harsh.  But it still would have been there.  The TC is not a great movie.