I don't think any superhero movie HAS to gross a billion to be considered succesfull, but I also don't think it was too high an expectation considering it had a 250k budget, starred the trinity or superheroes most known to mankind at the time Superhero films are in their most popular, the great precedent we had with Nolan's Batman films, and was supposed to kickstar the whole DCU.
But on the flip side there was a lot of shit being thrown at BvS before the first thing was known about the movie itself because of Ben Affleck taking over as Bruce Wayne. MoS did OK, but not great, and as far as DC movies go we really only have the Bale trilogy as a recent big success.
When you look at the Superman movies, what you mostly have is OK to poor box office results. The only amazeballs performance was the original 1978 movie that made $134 million in the USA, equates to >$400 million in today's money n the USA alone. Superman II did almost as well, then Superman III was disappointing and Superman IV was a failure. Superman Returns failed to break even, which is why it never got a sequel. Man of Steel only just made it to profit at the box office, and possibly wouldn't have got a sequel without the plan being to create the DCU to rival the MCU.
And a re-boot of Batman was far from a sure thing. I think people who expected $1 billion as a minimum KPI had far too high expectations. If the movie had been as critically praised as the Avengers movies I think $1 billion was a lock. But with it's relatively poor reviews I think nearly $900 million is an amazing result. It actually suggests that if WB/DC can make a great movie next up then $1.5 billion is definitely possible. So really the box office performance of BvS should give WB quite a bit of confidence.
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
"When the power of love overcomes the love of power, the world will know peace."