Why is Randy Johnson retiring?

Earlier today, it was reported that Big Unit is retiring. My question is why.

Johnson reached the superfluous 300 win benchmark last season with the Giants, all but solidifying his resume for the Hall of Fame. Johnson achieved this feat while posting a gruesome 4.88 ERA (96 IP) and spending half the season on the DL. Perhaps, at age 46, it makes some sense for Johnson to retire. He is all but guaranteed a Hall of Fame spot; he is comfortably rich ($100.2 million in salary from 2002-2008); he has a family; he is aging and injury prone.

However, there are two great reasons for Randy Johnson to keep playing. First and foremost, Johnson has 4,875 career strikeouts and is a stone's throw away from the 5,000 K plateau (a feat so far only accomplished by Nolan Ryan). Secondly, and most importantly, Randy Johnson is still very good at pitching.

Put the 4.88 ERA and 1.33 WHIP out of your mind. Look below the surface. Sure, Johnson's strikeout rate slipped to 8.04 per nine, his lowest mark since 1990, and sure, Johnson HR/9 spiked to 1.78, despite the move from Chase Field to AT&T Park. However, Big Unit's peripherals remain strong, even if he has not been effective as he was a decade ago. Big Unit's 8.04 K/9 rate ranks well above the MLB average of 6.59 per nine, Johnson's walk rate (2.91 BB/9 last season) is solid (MLB average is 3.46), and his GB% spiked by five points last season (45.4% GB%, 1.28 GB/FB ratio).

So why the poor season? It was not the BABIP (.297 last season, .303 career) or the LOB% (72.5% last season, 71.9% MLB average). Rather, Johnson's bad luck came in the form of long balls. Despite the fact that Johnson moved to a home run suppressing park this season and that Johnson burned the most worms he had since 2002, Big Unit's HR/9 rate soared. His 19.2% HR/FB mark was the second highest the majors amongst all pitchers who tossed 90+ innings in 2009. Randy Johnson's 3.79 xFIP* (a metric that analyzes a pitcher's defense-neutral pitching ability based on strikeouts, walks and xHR's [adjusted to a league average HR/FB ratio based on a pitcher's FB%) last season paints a much more optimistic picture of what Randy Johnson is still capable of than his ERA.

*xFIP clearly has its limitations, as a league average HR/FB rate does not account for park dimensions. However, it is a better predictor of future success for a pitcher than FIP or, god forbid, ERA. The theory behind xFIP is that pitchers have some control over how the batter will make contact with the ball (thereby inducing GBs and FBs), but that once the ball is out of the pitcher's hand, he has no control over where the ball lands, the defensive positioning or ability of the fielders, or even how strong of contact the hitter will make with his offering (and thereby how far the ball will travel).

Plain and simple, Randy Johnson is still very good. He is not the 1993-2004 Big Unit we will all remember, but he is still a quality #2/#3 guy. It is quite surprising to me that Johnson is not at all interested in reaching the mythical 5,000 K plateau. Whereas many pitchers (relatively speaking) before him have reached the 300 win mark, only one other has even touched the plane of 5,000 strikeouts. At 8 K/9, Johnson would only have to toss 141 innings to reach the 5,000 strikeout mark in 2010. If his innings were to be managed properly (a la Rich Harden), Johnson could surely stay health enough to attain this achievement.

After 22 seasons of effective pitching, Johnson retires atop his game. He's not the best of the best anymore (that reign has been passed on to teammate Tim Lincecum), but his career did not fizzle out. Randy Johnson will go down as not only one of the best pitchers of his generation, but as one of the best pitchers of all time. He has achieved more than almost everyone else and is guaranted as a first round Hall of Famer. Still, it's a shame he just won't get this one last achievement.