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.