In the years before prestige drama, characters didn't often die. To this day, cast members are still rarely culled from sitcoms. One of the things that made Cheers great was that the show never shied away from a harsh dose of reality. A lesser show would've replaced the late Nicholas Colosanto with another older actor or simply declined to reference the empty spot behind the bar in season four.
After Colosanto's passing, showrunner James Burrows said, "We're a realistic show, and we will deal with what happened to the coach in a realistic manner." True to his word, Burrows brought in Woody Harrelson to play a different role that was in the spirit of Coach; he wasn't a replacement, he was just a new employee. Prior to introducing Woody, the show made it clear that the Coach had died and showed the cast coming to terms with that.
Coach's death and Woody's casting didn't save the show; by that point Cheers was a hit and didn't need saving. But, the move showed viewers what made the show unique. The dark edges of the series are what make it timeless. In real life, bars have regulars, but people move, they stop hanging around, and sometimes they die. Live-action shows don't often run for 11 seasons. With the way Cheers handled Coach's passing, the show discovered how it would effectively present the departures of Shelly Long and the addition of Kelsey Grammar as Frasier Crane—how it could exist for the long haul. Unlike most sitcoms, Cheers understood that just because life goes on doesn't mean that it doesn't change—and the writers understood that that's one of the most wonderful and painful things about life.