Hershiser did not duplicate that year (who could) but he suffered a severe shoulder injury that threatened his career. Hershiser did come back and was an effective pitcher but he never again dominated and did not reach All-Star levels again. Still with a career with 200 wins and 2,000 strikeouts he had a record that was envied by most.
The issue for the Hall is that Hershiser was considered the best Starting Pitcher in baseball for a two and a half year period and a solid number two or three man after his shoulder surgery. If Hershiser was a dominate ace for a longer period of time his 1988 season would have helped make him a serious contender for the Hall.