John Walker IS worthy.
Reason: John Walker represents the current state of America's soul. Steve Rogers represents the height of America's Ideals.
In TV Tropes, there is a trope called Eagleland (guess what country they refer to).
It comes in two distinct flavor:
America the beautiful, cultured place where everybody strives for and defends freedom and justice, extended to everyone on the planet.
America the violent, chaotic, self-righteous world police who expects everyone to fall in line and submit to the american way of life.
Both are America. So in a sense…USAgent and Captain America are both worthy of the shield.