Just because an actor hasn't been featured in such a role that is widely known, doesn't mean its not outside of their capabilities (I will continue to point to Chris Evans going from Johnny Storm in FF to Captain America in EVERYTHING as the shining example of this and add James McAvoy in SPLIT as a more pertinent example). Yuen must have shown tptb something that made him fit the Sentry they wanted beyond being a blind diversity hire. The latter part is important - the Sentry they wanted. Removing Yuen from the role doesn't mean that whatever they have in mind will be scrapped leading to a lifted right from the page version of the Sentry character.