There's a strange little dynamic: during the drugs war both capos were found to be past their prime, and Vito dies several years later. So Michael might have welcomed the chance to replace either one. Their were risks in replacing either, because they had their own loyal people under them, so removing one capo would almost certainly require the assistance of the other.

The book is pretty melancholy about the family leadership. Vito is slippin', he knows Sonny isn't focused enough to be his heir, Tom isn't a war time consigliere, the capos are too old to command.