Quicksilver and Scarlet Witch have the perfect solution for Avengers: Age of Ultron. The one benefit to having Magneto for your father. "I can’t be the only person who has thought how much easier that fight would be with dadneto come on"
Well...I can definitely see why this is starting to become a thing because, to this day even in the freaking comics, we don't know how Mjölnir chooses who is friggin' worthy. But I'm not big on this because my own headcannon states that Mjölnir can say you're potentially worthy, but still not be quite there. Steve WILL be able to hold Mjölnir eventually, but as of right now, it's either not the right time or Steve still has some 'prove your worth' to do.