That's an incredible 71.4% for those keeping track. Here's a list of things less certain than Brady reaching the AFC Championship game.
Any other team in the NFL winning a regular season game in the Brady-era
The Patriots have won a league best 75.8% of games since 2001, so they're above the Brady-mark. 31 other teams have winning rates less than 71.4%. The Indianapolis Colts rank second with a 65.8% winning percentage (and yes, that's 10% worse than the Patriots).
The single season completion rate record
Saints quarterback Drew Brees holds the single season record for completion rate at 71.2%. This means that Brady is more likely to reach the conference championship game than any quarterback is to complete a pass.
Brady completing a pass to anyone other than Kevin Faulk or Wes Welker
Faulk (75.7%) and Welker (72.6%) are the only two players in the Brady-era to post a catch rate better than Brady's conference championship rate. Danny Amendola (68.9%) is the highest on the active roster, closely followed by Julian Edelman (67.8%). Rob Gronkowski is 9th on the list at 65.6%. Brady is more likely to reach a conference championship than to complete a pass to a member of the active roster.
But not Peyton Manning failing to reach the AFC Championship game
Manning has been a starting quarterback for 16 seasons (not including this season). He has made 4 conference championships. This means that he has failed to reach the conference championship on 75% of his completed seasons. This means that Manning is more likely not to reach the conference championship than Brady is to reach it.
What are some other things less certain than Brady reaching the AFC Championship game?
NFL Playoff Schedule