neither cheadle or whitaker are charismatic enough to pull off what denzel did in that movie. cheadle is a good enough actor, but his acting isn't electric enough to pull of what denzel did as alonzo.
cuba is just a laughable suggestion.
samuel l is charismatic enough, but not as activating and would not be as believable as denzel's portrayal of alonzo.
sam l. COULD have done it, we're just so used to seeing him be over the top that the believe-ability wouldn't have been there.
denzel makin snakes on a plane
sammy has his nitch, him in training day would have damn near been comedy.