The last 2 years suggest audiences are getting more selective about what MCU titles they'll support.
There was hype for Deadpool and Wolverine because both characters were still popular and there was a novelty in having them paired together. Guardians 3, like Black Panther 2, benefitted from the appeal of those particular characters/stories.
Ant Man 3, The Marvels and this? People aren't going to show up and like everything anymore. Giving Marvel the benefit of the doubt has been eroded by titles like these and all the Sony stuff.
I still expect Fantastic Four to do well, as they haven't had a movie in a while, but Thunderbolts may not. Especially since it's the next release after Captain America 4 and doesn't have any of the major, popular characters.