I definitely thought they'd make Bucky into Captain America when he was introduced as a redemption thing but after the sibling-ish relationship they showed in Civil War I think the dynamic is better served by Sam being Cap.
Hopefully in universe they have ways of keeping his normal person body ok.
Both have been Cap at different times in the comics iirc and they've broken away from those so much anyways that people should just get over it. Marvel movies are fun.
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