John Walker reminds me of the soldier Tommy Lee Jones’s character wanted for the super soldier serum in the beginning. The doctor in charge of the experiment said no. For a reason.
Steve Rogers grew up scrawny, knowing what it was like to be beat up, and not attracting much female attention. John Walker grew up getting girls and excelling at all things physical. The two different upbringings yielded two very different young men. When Bucky asked him if he jumped on a grenade and he said yes, with a reinforced helmet.... he’s not taking the same instinctively selfless risk Steve did. Walker’s sense of entitlement is on show so much throughout the most recent episode... I don’t think he’s as good of a guy as he appears to be at all. In fact, wouldn’t be surprised if in time, he turns out to be a bad guy.
The shield is made of vibranium. Concrete and high speeds wouldn’t do anything to it, except maybe chip the paint. No risk or sacrifice was made by throwing it for his buddy.