The shield most certainly could have been handed to Bucky if they had wanted that. Bucky has been on an arc of redemption and restoration, he knows who he was and where his loyalties lie. Sure, he may still have some issues, but in my opinion that makes him a more interesting and compelling character that we can all relate to because most people in the human race have some issues too. A psychological healing or transformation can be written, also something people experience. It's easier to connect with that emotionally than to an elevated paragon of virtue and purity. You can build drama around internal struggle, it's an opportunity for a writer. There's some mystery in his character which can also be explored.
Abilities at least partially define who or what Captain America is alongside character. The super soldier serum gave Steve the power to uphold the best American values. Sam doesn't have that. If they find a way to empower him in a similar way, it will be easier to roll with this, but what I fear is that Falcon tech will just be moved to a ground assault version. We already have Warmachine (or the new Iron Man?), "Iron Spider", "Iron Panther", and I wouldn't be surprised if Wakanda (recent allies, who are sharing their tech with the world) outfits Sam too. Then we'll essentially have another Black Panther but with a shield. I'm not looking forward to it.
Though I still believe that Bucky is the better option of the two, I can agree that neither are ideal. If I were Feige or some Disney exec pulling strings, Sam would remain the Falcon and they would hold off on Captain America, release their planned sequels on schedule, then reboot Cap along with the entire MCU (X-Men and the Fantastic Four included) in the next decade.