The Mick in his down, tack-on seasons 1965-1968: led Yanks in OPS+ each season: 137, 170, 149, 142 (career 172). In Mantle's last up season (1964) he led AL in OBP, OPS, OPS+, IBB; second in MVP (to Brooks Robinson) for the third time (also 1960, 1961 both to Roger Maris).
Joe DiMaggio in his one down season (1951) was SIXTH on Yanks in OPS+ with 115 (career 155) behind even the rookie Mantle's 116. Gil McDougald led Yanks in 1951 with 141. DiMaggio had a precipitous drop off from his 1950 OPS+ of 151.
Supposedly fellow San Franciscan and Yankee teammate Billy Martin convinced DiMaggio to play one more season after Joe D. led AL in slugging average in 1950. The only other time that DiMaggio was AL leader in slugging average was 1937 when he hit 46 home runs: 19 home, 27 road.
In 1951 DiMaggio had 12 homers and 36 strike outs, which indicated that he was finished. Had DiMaggio not played in 1951 he would have finished with more career homers than strike outs: 349 to 333. DiMaggio would have been the only slugger to do so.
In 1968 Mantle batted only .237. AL BA: .230.
In 1951 DiMaggio batted only .263. AL BA: .262.
DiMaggio was a shade above the league BA in his final season. Mantle was 3% above. However, because Mantle's BA was so low by normal standards he was considered to have been much less effective despite his 142 OPS+, which was 8th in AL, compared to DiMaggio's 115.. By current standards Mantle had a much more productive final season than did DiMaggio. Mantle could have continued. DiMaggio could not.