Koufax exited before his elbow blew out from arthritis. He hurt in originally sliding into second base in 1964. After that he was no longer so overpowering and doctors told him in 1966 he'd do permanent damage to his arm if he continued pitching.
Some suspect the baserunning play merely exposed something building in the elbow well enough before. (Some think
his straight-overhand delivery, especially throwing his curve ball, may have been a root cause.) He was also
told by the Dodgers' team doctor Robert Kerlan to retire before the 1965 season, in fact. It was Koufax's own decision
to call it a career after the 1966 season; he told no one except a
San Diego Union reporter to whom he was close
in spring training, Phil Collier, and Collier agreed to keep his mouth shut about it until the season ended. (Collier's
colleagues in the press were staggered that he kept the secret that long.)
As for "no longer so overpowering?"
* He went out in 1965 and won 26 games, struck out 382 batters (a major league record until Nolan Ryan broke it---by one),
won the pennant clincher on two days' rest, then won two World Series games including Game Seven on two days' rest. (This
was the Series in which he refused to start Game One because it fell on Yom Kippur.) Not to mention a fourth no-hitter,
which proved practise makes perfect: his perfect game, his fourth no-no in four seasons.
* He also pitched better than his 2.04 ERA---his fielding-independent pitching rate was 1.93, with an 0.86 WHIP.
* He led the majors in strikeouts per nine (10.9) and with a 5.38 strikeout to walk ratio.
* He won his second of three Cy Young Awards won in a time when the award went to one pitcher across the board
instead of one in each league.
* In 1966, he won 27 games, struck out 317 hitters, had a 1.76 ERA, and his 2.07 fielding-independent pitching led
the majors for the sixth straight season. He was forced to win the pennant clincher on the final day of the season,
on two days' rest yet again, when Don Drysdale got clobbered in the first half of a doubleheader against the Phillies
and forced manager Alston to change his intended plan and---instead of holding Koufax until Game One of the World
Series---pitch Koufax in the nightcap to clinch.
* He had only one 1966 World Series start and was pitching up to par until three errors in the fifth inning undid him
and the Dodgers; the Orioles went on to sweep before Koufax could get another crack at them. (He was tapped to
start Game Five if there was one.) He retired with a lifetime 0.76 World Series ERA.
* He won his third one-across-the-board Cy Young Award.
It seems to me that Sandy Koufax---with an arthritic elbow for which he was medicated heavily enough to be a little
high on the mound and possibly prone to line drives back through the box (he cited the possibility of being injured
by one under that insane medical regimen at his retirement announcement)---was overpowering enough and then
some in his final two seasons. It's not that you'd recommend it to anyone else, but for two seasons with a condition
that could have crippled him for life and under meds some of which could have killed him (yes, you can look it up),
Koufax went from off the charts to the tenth dimension. And retired not at the top of the heap but beyond it.
