I've found out more information regarding WWE changing the Survivor Series main event. The show was originally scheduled to feature a Traditional 5-on-5 Survivor Series Match with John Cena & The Rock anchoring the babyfaces with The Miz & R-Truth anchoring the heel team. The plan was changed when it was discovered Mick Foley had a scheduling conflict and would most likely be unavailable for the pay-per-view. I'm told by a source with direct knowledge of the situation that a lot thought was given as to who his replacement would be.
The concern was that without a third major name to add to the team of The Rock and Cena, having five guys on each side might just end up "watering things down." Vince McMahon went directly to The Rock with the idea of simply doing Rock & Cena vs. Miz & Truth to see if he was comfortable with it. Remember it was The Rock's idea to compete at Survivor Series as he wanted a chance to get his feet wet in a match involving ten guys before Wrestlemania XXVIII next year. I'm told The Rock ended up being in agreement that without Foley, the Traditional 5-on-5 Survivor Series Match wouldn't have the same "oomph" as just going with the tag match.
This ended up coming as a relief to the writing team as many of the writers felt there was a lot of pressure to stack talent in that match and with ten guys, they feared the undercard would end up very thin. Especially now that it looks like WWE is delaying Triple H vs. Kevin Nash to their Tables, Ladders & Chairs pay-per-view next month.