Girls leaving that caused the groups to go poof.
It is true that through fault of their status in the group or sheer bad timing that certain girls who left the group while at its peak would cause the group to go belly up cases in point
Florence Ballards leaving in 1967 caused a downfall of records sales from 67 through 69.
Betty Kelly leaving the Vandellas caused its last top ten single "Jimmy Mack" "Honey Chille recorded in 1967 was released after Betty left and it only hit #11. their last top twenty single with Lois Reeves.
Gladys Horton leaving the Marvelettes in 1967 as noted on tvone caused the group to unravel, its last top ten was released in 1966 DMWB before Gladys left even though she was not the lead singer, after she left the group recorded no more top ten pop hits: Hunter #13, YAIL #23 MBMBAM #17 though the records did better on the r&b charts.
And Finally after Cindy Birdsong left the Supremes in 1972 they had recorded their last top 20 single with her on bcg vocals. their popularity was vastly affected with her departure. .Unfortunatley with her replacement no more top hits would ensue, although Scherrie would chart a top 40 hit in 1976 for the group.