This thoughtful thread points out a real-life reality for Mob bosses: they can never really be totally sure of anyone around them,can never totally trust or take the loyalty of anyone around them for granted--and probably never love too many people because of the real possibility that the loved one will either be killed or betray them. Clemenza was, with Tessio, Vito's oldest retainer, but when Vito was shot, the finger of suspicion pointed at him first. And even though it was Sonny who pointed it, Vito probably would have, too, if he'd been conscious.
OK, Tessio stayed loyal to Vito, but betrayed Michael after Vito died. But, after the Tahoe shooting in II, Michael tells Tom that he can't be sure of his "people because they're all businessmen, their loyalty is based on that. And that includes Neri, who (unlike Tom) was Michael's choice from the beginning.