It's Marvel, owned by Disney, regardless of who plays it there will be money behind this.
And unlike some of the names dropped, MJW has an extensive acting career and is pretty much already physically fit for the role. Also, even if Spawn was shyt, his role in TDK was small and they wasted him in Arrow, it still gives him a namesake for being invested in comic book properties.
Only thing working against him is time since he isn't getting any younger, but at best we can always hope he plays T'Chaka so he can give young T'Challa some wise life lessons and show him what he must aspire to be.
It has to be an actor with a name and some chops. You can't sell the movie with him. What if Panther is in the next Avengers movie? We got:
Downey Jr
ScarJo
Renner
MJW
It would be nice if we had more than 5 black actors to choose from. Idris would kill it but he's in Thor. Chiwetel is on fire right now so he'd be my pick.