Alot of reasons dude's career has been trash for the past decade, and it ain't got shyt to do with the industry trying to get a black dude out the paint.
Dude decided a long time ago what type of movies he was gonna do. Either a big budget, sci-fi/action blockbuster, a drama he could potentially win an Oscar for, or a film that he could produce himself and have a great deal of creative control over. Every movie he's done in the past 20 years has fit in one of those three categories. (Except Focus)
His blockbusters as of late haven't met initial studio box office estimates for DOMESTIC gross. However, they still do very well overseas, in large part due to the fact that Action Sci Fi films that don't have elaborate plots perform better overseas than any other type of American film. But they're still seen as disappointments as far as North American box office.
His Oscar chasing roles have been in good, but not great films. He's taken safe, image conscious roles, and he works with shytty directors. His first 4 starring roles were all hits, which probably created unrealistic expectations in people's minds. Not alot of non-comedic actors that you can just throw in any movie and have it be a huge hit. Actors gotta pick the right roles and films. Will Smith's done a bad job of that for over a decade now.
You've gotta pick the right projects to flourish. Eddie Murphy was flourishing. He started doing shytty movies and fell off. Jim Carrey was flourishing. He started doing shytty movies and fell off. Adam Sandler was flourishing. He started doing shytty movies and fell off. Arnold, Stalone, Nic Cage, Travolta, Affleck until recently, Keanu, etc. List goes on and on.
Not alot of guys that've been in the game as long as Will and not fallen off for an extended period of time. You can't expect every movie he makes to do Independence Day numbers. But its not too much to ask for him to choose quality projects instead of just following a formula. Denzel consistently picks dope roles. Leo consistently picks dope roles. Sam Jackson and Jamie Foxx too.
I think Focus was a step in the right direction, even tho it was just an alright movie. Concussion should've been put together better, but you can't expect a movie like that to do huge numbers no matter who stars in it. The subject matter is boring. Suicide Squad seems like a good look, now he needs to follow that up with Bad Boys 3, and build from there.
the 90's were a loooong time ago...big willy just ain't the box office draw he once was
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. Will decided himself to fall back and start producing and focusing on dramatic roles as he's getting older. Just because he's intentionally not making the films you want him to make, it doesn't mean his career is dying 
. He's doing exactly the same as Angelina Jolie.