If Mickey Mantle had retired after 1964, he'd be an even bigger legend.

baseball-reference.com has this cool "Cumulative Batting" under "More Stats" for a player.  It has the data by year and here it is for Mickey Mantle through 1964, Mantle's 14th season when he was still only 32 years old.

Mickey Mantle 1962 Baseball Guide and Record Book by J. G. Taylor Spink [Public domain], via Wikimedia Commons

Year	Age	G	PA	AB	R	H	2B	3B	HR	RBI	SB	CS	BB	SO	BA	OBP	SLG	OPS	OPS+	TB	GDP	HBP	SH	SF	IBB
1951	19	96	386	341	61	91	11	5	13	65	8	7	43	74	.267	.349	.443	.792	117	151	3	0	2
1952	20	238	1012	890	155	262	48	12	36	152	12	8	118	185	.294	.377	.497	.874	144	442	8	0	4
1953	21	365	1552	1351	260	398	72	15	57	244	20	12	197	275	.295	.384	.497	.881	144	671	10	0	4
1954	22	511	2201	1894	389	561	89	27	84	346	25	14	299	382	.296	.391	.505	.896	148	956	13	0	6	4
1955	23	658	2839	2411	510	719	114	38	121	445	33	15	412	479	.298	.400	.528	.928	155	1272	17	3	8	7	6
1956	24	808	3491	2944	642	907	136	43	173	575	43	16	524	578	.308	.412	.560	.972	166	1648	21	5	9	11	12
1957	25	952	4114	3418	763	1080	164	49	207	669	59	19	670	653	.316	.427	.574	1.002	174	1963	26	5	9	14	35
1958	26	1102	4768	3937	890	1238	185	50	249	766	77	22	799	773	.314	.430	.577	1.006	176	2270	37	7	11	16	48
1959	27	1246	5408	4478	994	1392	208	54	280	841	98	25	892	899	.311	.425	.569	.994	173	2548	44	9	12	18	54
1960	28	1399	6051	5005	1113	1537	225	60	320	935	112	28	1003	1024	.307	.422	.568	.990	172	2842	55	10	12	23	60
1961	29	1552	6697	5519	1244	1700	241	66	374	1063	124	29	1129	1136	.308	.425	.579	1.004	175	3195	57	10	13	28	69
1962	30	1675	7199	5896	1340	1821	256	67	404	1152	133	29	1251	1214	.309	.429	.581	1.009	176	3423	61	11	13	30	78
1963	31	1740	7412	6068	1380	1875	264	67	419	1187	135	30	1291	1246	.309	.429	.582	1.011	177	3530	66	11	13	31	82
1964	32	1883	7979	6533	1472	2016	289	69	454	1298	141	33	1390	1348	.309	.429	.582	1.011	177	3805	75	11	13	34	100

In 1965-1968, Mantle's final seasons, neither Mantle nor the Yankees did much that was especially noteworthy. The Yankees did not even contend for a pennant.  Division play began in 1969, the year after Mantle retired.  Mantle would not have been tarnished with those losing teams after being on 12 pennant winners.  The demise of the Yankees would have been directly attributed to the absence of Mantle.

The only major milestone reached by Mantle was home run number 500 in 1967.  All things considered, The Mick would have been much better off by leaving while he was still at or near the top of the American League.  Mantle already had three MVP awards (1956, 1957, 1962) and three seconds (1960, 1961, 1964).

Mantle broke his tie at 15 with Babe Ruth for career World Series home runs with a walk off winner in game three, then homered in games 6 and 7 with his team facing elimination in each game.  WS homer 18 was off Bob Gibson.  With the Yanks facing elimination in game 4 of the 1963 WS Mantle homered off Sandy Koufax in the 7th inning to tie the game 1-1.

Mick had 454 home runs through 1964, one more than Willie Mays in many fewer at bats (AB): Mays 7,036, Mantle 6,533.  In 1965 Mays hit 52 homers to zoom past Mantle.  In retirement, there would have been the speculation, no matter how unrealistic, that Mantle might have matched Mays.

Mantle's batting average would have remained well over .300: .309.  His OPS would have been over 1.000: 1.011.  Mantle's slugging average would have been higher than that of Joe DiMaggio (.582 to .579) and the length of their careers much closer: plate appearances: Mantle 7,979, DiMaggio 7,673.