As to Rob, I was not implying that he was forced out. You made the comment that Jim hasn't declined over time. I assume you were referring to his presentation. Jim is 65. I was simply stating that I believe Rob is in his upper 60s (youngest kid is a senior in college) and that he hadn't declined in his presentation either (despite the fact that you think he has in 2012), but chose to retire despite still having his A game. So it is possible that Jim could decide to retire too for the sake of enjoying retirement, even though he still has his A game.
As to Marc's replacement, Rick and Monica were both in the mix to replace his as was Rob. I believe they didn't want to break up Rick and Monica, so towards the end, the feeling was that Rob was the leading choice, especially since he was already the primary fill-in for 5,6, and 11, and also had more seniority. I remember reading something at the time, I believe perhaps written by Laura Nachman, but can't find it now.
Lastly, it is laughable that they would ever consider Rick or Brian for the weekend. That would clearly be a demotion!