Extro

Those cases were different. The original Rodey quit I think, and they recast Banner because they originally want Mark Ruffalo but he wasn't available for the Hulk movie. He was for the Avengers, so they got him.

Don Cheadle was definitely an upgrade for Rhodey, and Terrence Howard left because they couldn't negotiate the paycheck. At the time, War Machine wasn't nearly as popular as he is now.

There's a story in the comics where Shuri becomes the ruler of Wakanda and the Black Panther herself. Maybe they could say T'Challa got lost out on some recon mission? Either way, Chadwick left T'Challa's shoes real big, and I dont think it's possible to get anyone to fill them like he di.
Ngl though, maybe Kevin wouldn't recast but I wouldn't put it past Disney. They're all about the money, the story doesnt matter to them

I think perhaps they could make the movie about the kings mysterious disappearance. Then they need to come up with a worthy way for his character to die. They could do a death scene even.  The suit was CGI half the time anyway.