"... how many companies have been on top for 30 years?" Looks like General Motors and AT&T were top 2 from 1930 to 1960: https://taaginc.com/large-and-in-charg

"... how many companies have been on top for 30 years?" Looks like General Motors and AT&T were top 2 from 1930 to 1960: https://taaginc.com/large-and-in-charge/ https://taaginc.com/large-and-in-charge/