The most natural comparison to make in the story of Calvin Johnson — who reportedly told Lions teammates before the 2015 season that he planned to retire at the end of the year and still seems to be on that path — is of course to fellow Detroit star Barry Sanders.
Sanders retired abruptly shortly after turning 31, an age Johnson will reach in September. Both were still very productive. And just as Sanders' retirement left a void for the Lions both on and off the field, so too would Johnson if he indeed calls it quits.
My first instinct whenever an athlete retires with plenty of good playing years potentially ahead, though, is to think of a local athlete: former Vikings running back Robert Smith.
In case you have forgotten, it was almost exactly 15 years ago when Smith — coming off his best season in the NFL with 1,521 rushing yards for the Vikings — decided to retire just a month before his 29th birthday (even younger than Sanders or Johnson).
Part of Smith's motivation was a knee problem that would have required more surgery and rehab to fix. But some of it, as reported at the time, was a simple premeditated desire to move on to another phase of life. Per a February 2001 Star Tribune story quoting former Viking Chris Walsh:
"He was talking about  being his last year," Walsh said. "I was perplexed at the time. After the season … he was pretty sure that's what he was going to do. … He said, 'The thing that's sad to me is a guy who stays too long.'"
Smith walked away quietly, by e-mailing a statement to a reporter from the Cleveland Plain-Dealer who had covered him for many of his amateur days.
Sanders is a Hall of Famer, and Johnson will be one. Smith is not. But still, the impact on the Vikings was palpable. Though hardly the only factor in a rapid decline, Minnesota went from the NFC title game in Smith's final season (hello, 41-donut) to 5-11 in 2001, a season in which Dennis Green was fired as head coach. The Vikings, in fact, only made the playoffs once in the seven seasons after Smith retired, and they struggled for years to find stability at running back.
The Lions, who made the playoffs five times with Sanders, actually went 17-15 combined over the two seasons after Sanders retired and made the playoffs once. But they really bottomed out in the 10 seasons after that — missing the playoffs every year and sustaining double-digit losses in all but one season.
Detroit went to the playoffs twice with Johnson — though the Lions' failure to go more often can't be pinned anywhere near the gifted wide receiver. If we have, indeed, seen the last of him, it is safe to say the impact on the Lions will be profound.