The problem is, the budgets on these films are now so routinely huge, that they actually need to hit $1 billion to be considered profitable. Once a studio has spent $200-300 million on production, than another $200 million on marketing, they're out of pocket $400-500 million, and conventional Hollywood accounting is that they need to make 2X that figure to make money, since the theatres take a cut etc.
Marvel used to be fairly tight-fisted, with movies costing around $150 million, and a smaller marketing budget, meaning they could be considered profitable on a return of $500-600 million worldwide.
So with regards to opening weekends, they're fairly reliant on a big opening weekend to carry the movie. Most films, even Avatar 2, Force Awakens & Infinity War usually see a drop-off of about 50-60% in their second week, compared to the first.
So if a movie only makes $200 million globally in it's first week, they can expect to see that number halve in it's second week, which can spell disaster when you need to claw your way to a billion to make your money back. Conversely, Endgame was the first movie to pull off the mythical "Billion Dollar Opening Weekend", which meant it was still pulling in $500 million in it's second weekend, with that 50% drop off.
