While it would certainly be unavoidable to have at least subplots and subtext about Sam, vis a vis race and government control, as an element, there is at least one reason to be skeptical about Sam-as-Cap that isn't obviously bogus: the Super Soldier Serum. Steve had superpowers, and Sam does not. It is at least prima facie reasonable to go "Your a great hero and all, but the next Captain America needs to have similar powers, not just be an ordinary skilled guy with some gadgets".
Of course, I suspect this would be at least partially a pretext to get a Captain America that the relevant government parties can better control, with at least some people going further with a subtext of "Cap needs to be white". However, this only really works if there is at least basic plausibility.