There are three established roles for Jim Halpert to play in The Office. The first, and most consistent in this series and the BBC version, is that he is the ongoing office rival of the Assistant To The Regional Manager. Over the years, the laughs derived from their interactions have been on a regular basis. The second role, is that of the straight man, who also provides the voice of reason for Michael Scott. There have been some changes to this role, mostly when Jim had to step up in Michael's absence, or during his brief stint as co-manager. In these hectic times, Jim learns just how little he knows, while the ominous foreshadow of him turning into Michael is something he finds horrifying haunting. The third role that Jim plays, that has also happened to change the most over the years, is his relationship with Pam Beesley. During the first three seasons, Jim was basically tortured on an emotional level, as he hopelessly longed for Pam. Over these three seasons, Jim's crush on Pam contributed to a lot of the painful and uncomfortable humor that the show became known for, and made Jim into an everyman that was easy for the viewer to identify with.
Since getting into a relationship with Pam, Jim no longer has the role of pursuing the office's receptionist. With this simple, yet cornerstone plot element removed from the series, the writers had to create new storylines for Jim that still involve Pam. In this process, Jim has become significantly dumber and neurotic over the years, for no apparent reason, except that the writers need Jim to be doing something comical on the show this side of pranking Dwight or making bad management decisions. With the introduction of baby Halpert, Jim has been constantly fumbling his way through his parenting, making him differ significantly from the man that he was before. This week's episode, Christening, was a prime example of Jim's deteriorating intellectual health. In his attempt at locating his daughter, Jim acted so thoughtlessly, that he now regularly challenges Michael as the dumbest man alive.
But taking Michael's crown is no easy task. Like in Phyllis' Wedding, Michael found himself in a church once again having to deal with his reoccurring feelings of wanting to fit in. After seeing the camaraderie that was in the crowd of the young missionaries, Michael decided that he wanted to join in on their manual labour trip, despite the protests of his staff. He and Andy did eventually come around and realize that they were being unrealistic, and ask for the bus driver to let them off. Michael slowly learning to do the right thing is an arc that makes sense, whereas Jim becoming dumb for no reason is not. This also goes to show that the character of Michael Scott can not be replaced, as Steve Carell is a pure comic genius that has carefully spent the last seven years evoking every emotion in viewers. Granted, there will always be other characters and moments that provide the random laughs, such as Kevin catching Angela stealing scones, or Dwight attempting to sell printers and paper at the church, but the show would just not be the same without Michael Scott.