Averaging out all movies of the MCU since 2008 when you include all $1B+ movies and even Thunderbolts as of today and assume it made no more money, the average is $879M. When you take out the $1B+ movies and include Thunderbolts as of today again assuming it made no more money, the average is $589M. Even if you took out Thunderbolts because we assume it'll make more money, the average is $606M. So the range to me is $500-$650M on average is good for an MCU movie, and as
@Animaniac93-98 rightfully said $400M should be the floor.
The $1B+ movies are the outlier in the MCU, they aren't the average, they are the exception to the rule not the rule itself. They should not be the expectation, which is the whole point of this discussion.