I think the key with Sam becoming cap, is to show the one thing that made Erskine saw with Steve as the best choice for the serum in The First Avenger:
He wanted someone, to be not just a good soldier, but a good man. Continuously throughout the MCU, and especially here; we see Sam Wilson show that he isn't just that of the military. He would often resort to that of the diplomatic element, due to his service as a counselor for veterans from The First Avenger; and his chat with Karli in Europe.
He would rather ensure the safety of others, and to ensure the mission is done in a way that results in no-one getting majorly hurt. And while Steve did eventually get more brutal in some ways; he still maintained the good that Erskine had saw at the Stark Expo site. I really hope Sam doesn't get the serum, as it shows a much more human side to the matter (even more than Spider-Man, out of all the heroes in the MCU); in that he isn't a super soldier, or a witch; or a teenage who has superstrength and the ability to traverse through terrains regardless of the degree of the surface. He is human.