He's a Nostalgia Act. He draws because 2002 draws. Other numbers comparable to this involved Triple H and Brock Lesnar. This isn't even the highest average viewership in the past half a year. Austin/Punk would have drawn the same friggin' numbers. A few hundred thousand people still marking like children for AE/Ruthless Aggression stuff show up, watch that one little portion and then ultimately leave. The only long term payoff to this Era-milking is to devalue the present era and kill your viewership whenever you don't have somebody of a 2000 vintage show up.
Rock's return segment with Punk can be higher than a 3.6 if the overall rating is a 3.14(which is a huge increase that surprised me), but the peak of Rock/Cena in terms of TV audience was in 2011. And nothing is touching that for a long, long time if at all. Rock's first advertised return in Chicago did one of the most watched segments in the last decade. Rock/Cena in Atlanta after WM27 also drew a crazy audience with a 4.6.