#7 John Cena vs. The Rock — WrestleMania 29
(First match: Rock def. Cena — WrestleMania 28)
"Once in a lifetime" apparently only extends to one WrestleMania in WWE lingo. After promoting their first match as such, WWE had the nerve to book the same match-up as next years main event.
After The Rock pinned Cena at WM28 in the most heavily promoted match in WWE history, the path to WM29 was already set, and the WWE Championship had to be involved. Inbetween the two 'Mania's, CM Punk was setting records as WWE champ, and Ryback emerged as one of the hottest acts in years.
Instead of featuring either Punk or/and Ryback in the main event, WWE decided to not deviate from their set route. Punk lost the title to Rock and Ryback was tossed out of the Royal Rumble by winner Cena.
Neither Punk or Ryback got as popular as they were in that period. Punk walked out the next year. Ryback went through multiple restarts before tiring and quitting in 2016.
The Rock/Cena rematch failed to attract a quarter of interest the first bout garnered; the intrigue was not there. The match itself was good, but the outcome was never in any doubt. Cena defeated The Rock to win the WWE Championship
The image of The Rock endorsing a victorious Cena was planned two years previously but on the route to this 'WrestleMania Moment', WWE lost sight of their future.