When Babe Ruth retired in 1935, this is what the all-time home run list looked like:
Babe Ruth 714
Lou Gehrig 378
Jimmie Foxx 302
Rogers Hornsby 300
Note, only four players with 300 or more home runs. Gehrig was the first player after Ruth to reach 400 homers. In 1940, Foxx became only the second player to make the 500 club. Mantle topped him in 1968. When The Mick retired after the 1968 season, he was third on the all-time list with 536.
By 2002, Mantle dropped 10th on the list. He was replaced by Sammy Sosa (539) in 2003.
When Alex Rodriguez hits his 600th homer, only three players in in the top 10 (Brooks Robinson 586, Mark McGwire 583, Jim Thome 576) will have under 600 homers.
It is just a matter of time where 500 homers will not qualify as a top-10 achievement.