Don Cheadle's War-Machine is to Iron Man what Falcon is to Captain America: the trusted friend, the right-hand man, and a likeable character with a good sense of humour. Also, rather disposable. He performs a similar function to Iron Man, only in a more official government capacity, and not quite as well. With Sam Wilson, there is a possibility that he might become Captain America later down the line, but I don't think anyone looks at War-Machine and thinks of him as a successor to Iron Man. This is the sort of death that Marvel have gone for previously, a well-liked character, but not one with a major role to play in things, so he'd be a safe choice to kill off here, which might be part of the reasoning if it doesn't happen. That would be a bonus in another regard, because Cheadle - along with Anthony Mackie - bring some much-needed diversity to superhero movies, and they need more of that, not less.